Fix certifi fallback is not used on Windows.

Fix GitHub typo

Python 3.6 is out

[CI skip]

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.