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

Markdown is a widely used format to write README files and
documentation. This patch aims to simplify adding new formats and at
the same time adds that one to the list.

Correctly replace conflicting distributions in sub-requirements
if possible (instead of only for top-level requirements passed
as arguments).

Fix #1124.