Notify data managers that join transactions in progress

  transactions were in progress didn't have their newTransaction
  methods called to let them know of the in-progress transactions.

New transaction API for storing data on behalf of objects

And that it's id is used.

For objects such as data managers or their subobjects that
work with multiple transactions, it's convenient to store
transaction-specific data on the transaction itself.  The
transaction knows nothing about the data, but simply holds it
on behalf of the object.

Discussion:
https://groups.google.com/forum/#!topic/python-transaction/oUzj3uIHBgA

Free DataManagers when a transaction becomes non-current.

added ITransactionManager.registeredSynchs and clearSynchs

To support client testing.

Add support for 3.5.

This didn't work when @tseaver tried it for zope.component, but this is
a much simpler configuration. Maybe it will work here like it does for
BTrees?

Drop support for Python 2.6 and 3.2.

- 2.6 is long out-of-maintenance, and a security quagmire.

- 3.2 cannot be tested on Travis.

Also, point to RTD for docs.

[ci skip]

[ci skip]

Format log only when actually logging.

In WeakSet, use the `valuerefs` method from Python 2.5 instead of accessing the internal `data` implementation detail.

In WeakSet, use the `valuerefs` method, added in Python 2.5 instead of accessing the internal `data` implementation detail.

This lets us support Jython.

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

This was seen on winbot[*]:

  ValueError: path 'docs/_build/' cannot end with '/'

with a traceback ending in "c:\Python26_32\lib\distutils\util.py", line
201, in convert_path.

[*] http://winbot.zope.org/builders/transaction_py_265_32/builds/845/steps/buildout/logs/stdio

I cannot reproduce the error locally, using Python 2.6.9.

Also update MANIFEST.in to exclude files that shouldn't be in the sdist.

Also, work around edge case on Travis where files aren't found.