Python 3.6 is out

Remove installation instructions from the README and instead refer to the Python Packaging User's Guide, which indicates to use pip for installing setuptools. Moved Credits to the history section of the documentation. Cleaned up hyperlinks.

Fix broken link in docs/developer-guide.txt

Document use of environment markers in extras_require

PEP 496 is a draft that was never accepted, so it does not have the
correct information.

Add a no-op property for 'tag_svn_revision' to suppress errors when distutils attempts to detect and set these values based on settings in setup.cfg as found in sdists built by earlier versions of setuptools. Ref #619.

Add two more tests for _iter_code per #866, capturing the apparent expectation in the byte-code processing that's now failing on Python 3.6.

Attempt to fix issue #866 by iterating over code with `dis.Bytecode` instead of the internal `_iter_code`.

The `dis` module was already used in `_iter_code` so I figured it was safe to use `Bytecode` from it. Not sure how this assumption holds up across all supported Python releases. I can only assume `Bytecode` wasn't there before when `_iter_code` was originally written?

Note that `_iter_code` doesn't appear to be called anywhere in light of this change so I removed it.

I should also note that `get_module_constant` has never worked with `setuptools.__version__` (returns -1) because it's not a string literal; it gets that attribute from another module.  But this change does work in cases where a string literal is requested.

https://github.com/pypa/setuptools/issues/866

Intended for release to be 32.1.4, but I typed 'bumpversion minor --no-tags' instead of 'bumpversion patch --no-tags' by mistake, then tagged with what I thought I'd just bumped to.

Re-use test.paths_on_pythonpath to extend the PYTHONPATH variable rather than erasing it. When tests are run under pytest-runner (or other setup.py test invocations), the PYTHONPATH is carefully curated to include dependencies and the project under test. Overwriting PYTHONPATH will break tests in those environments. Fixes #884.