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.

Test that specifying a `long_description_content_type` keyword arg to
the `setup` function results in writing a `Description-Content-Type`
line to the `PKG-INFO` file in the `<distribution>.egg-info` directory.

`Description-Content-Type` is described at
https://github.com/pypa/python-packaging-user-guide/pull/258

This is used to populate the new `Description-Content-Type` field.

`Description-Content-Type` is described at
https://github.com/pypa/python-packaging-user-guide/pull/258

Allow combining several files in metadata.long_description

If one wishes to test both 'mixed' and 'sandboxed', let's create a separate test for that, but it's probably unnecessary.

The current code in find_on_path is doing a lot of stat() calls which are
actually useless and prone to race conditions.

As described in Python documentation
(https://docs.python.org/3/library/os.html#os.access), os.access must not be
used before opening a file. Same goes for a directory.

This patch removes those checks by handling exceptions correctly when using
os.listdir() instead, which improves pkg_resources import time.