Proofreading for the setup.cfg documentation

travis: make deploy stage conditional

fix `test_pip_upgrade_from_source` test on Python 2.6

Add badges to the top of README

* PYPI
* Travis CI
* AppVeyor CI
* List of currently supported Python versions

Fix Python 2.6 support

pkg_resources: do not call stat() and access()

Improve README file list handling and add Markdown to the current list

pkg_resources: improve `WorkingSet.resolve(replace_conflicting=True)`

workaround easy_install bug

Add new long_description_content_type kwarg

Sort file lists