Add a module that implements the PEP 517 specification.

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