Add support for 3.6, drop 3.3

[skip appveyor]

Remove outdated comment.

Remove redundant usedevelop.

[skip ci]

We need a newer setuptools on older versions of Python to be able to install recent Sphinx because of its use of environment markers.

Also add note on supported versions to the docs and readme.
Fixes #168.

Update tox.ini to correctly run testdocumentation and DRY with test deps.

* clean up references to DBs and Connections in pools
* Implement an explicit clear() method for clearing all the items in the pool.

* Make all classes new-style.

On PyPy, it's documented (http://pypy.org/performance.html#id3) that
old-style classes are slow, and classes that derive from both old and
new are especially slow. I checked with the PyPy devs on IRC today and
they confirmed that's still true with the latest PyPy2 releases.

Unfortunately, FileStorage was one such mixed class, as was Connection.

Moving to new-style classes seems to have a positive impact in the
benchmarks. Here's zodbshootout on PyPy2 5.7.1 against a
FileStorage (running in --threads and with 5 reps to be sure we get
warmed up). First, current ZODB:

"Transaction",                fs
"Add 1000 Objects",            31,738
"Update 1000 Objects",         42,444
"Read 1000 Cold Objects",      51,894
"Read 1000 Hot Objects",       53,187
"Read 1000 Steamin' Objects", 835,877

And with this PR:

"Transaction",                fs
"Add 1000 Objects",             35,651
"Update 1000 Objects",          54,906
"Read 1000 Cold Objects",      103,484
"Read 1000 Hot Objects",        84,721
"Read 1000 Steamin' Objects", 2,112,095

The tests that hit the storage extensively are notably faster, as are
steamin and hot, Connection having been a mixed class.

I ran all tests multiple times. The data files were removed between
runs. There's some variation, but the new-style classes always seem
better.

For comparison, here's CPython 2.7.13:

"Transaction",                fs
"Add 1000 Objects",            19,531
"Update 1000 Objects",         16,201
"Read 1000 Cold Objects",      22,111
"Read 1000 Hot Objects",       21,851
"Read 1000 Steamin' Objects", 880,582

Locally I've run the tests under 2.7 and they all passed.

Profiling (https://github.com/zodb/zodbshootout/pull/32/) showed that
this method was the only blob-related method that showed up in a test
of creating blobs, other than those that actually performed IO.

With this change its total and cumulative time drops from 0.003/0.004
to 0.001/0.002 in a small benchmark. Blobs created per second shows a
small but consistent improvement.

Before:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      100    0.005    0.000    0.005    0.000 {built-in method rename}
      100    0.004    0.000    0.004    0.000 {function BlobFile.close at 0x1080d3a60}
      200    0.003    0.000    0.004    0.000 blob.py:576(oid_to_path)
      101    0.003    0.000    0.003    0.000 {built-in method mkdir}
      100    0.002    0.000    0.002    0.000 blob.py:333(__init__)
      402    0.002    0.000    0.005    0.000 {method 'dump' of '_pickle.Pickler' objects}
        1    0.002    0.002    0.034    0.034 Connection.py:553(_store_objects)
      201    0.002    0.000    0.002    0.000 {built-in method stat}
     5633    0.001    0.000    0.002    0.000 {built-in method isinstance}

After:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      100    0.005    0.000    0.005    0.000 {built-in method rename}
      101    0.005    0.000    0.005    0.000 {built-in method mkdir}
      100    0.004    0.000    0.004    0.000 {function BlobFile.close at 0x10636aa60}
      402    0.002    0.000    0.005    0.000 {method 'dump' of '_pickle.Pickler' objects}
      100    0.002    0.000    0.002    0.000 blob.py:333(__init__)
        1    0.002    0.002    0.035    0.035 Connection.py:553(_store_objects)
      201    0.002    0.000    0.002    0.000 {built-in method stat}
     4033    0.001    0.000    0.001    0.000 {built-in method isinstance}
   ....
      200    0.001    0.000    0.002    0.000 blob.py:576(oid_to_path)

Fixes #158.

And emacs backups. :) Because of the way zest.releaser works, this
isn't strictly necessary, but it makes the manifest checker shut up.

* Fixed: A blob misfeature set blob permissions so that blobs and blob
  directories were only readable by the database process owner, rather
  than honoring user-controlled permissions (e.g. umask).

Tests running and passing on windows.

Fix PyPI page

Update RelStorage information in multi-zodb-gc document

FileStorage: Report problem on read-only open of non-existent file

... instead of silently creating empty database on such opens.

Use-case for this are utilities like e.g. zodbdump and zodbcmp which
expect such storage opens to fail so that the tool can know there is no
such storage and report it to user.

In contrast current state is: read-only opens get created-on-the-fly
empty storage with no content, but which can be iterated over without
getting any error.

This way e.g. `zodbdump non-existent.fs` produces empty output _and_
exit code 0 which is not what caller expects.