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.

to generate reproducible zip files
that do not differ depending on (random) filesystem order

See https://reproducible-builds.org/ for why this matters.

Now it's possible to use it like this:
* long_description = file: README.rst, CHANGELOG.rst, LICENSE.rst

* `file:` not accepts comma-separated list of filenames
* files' contents are glues with an LF separator

Rename [wheel] section to [bdist_wheel] as the former is legacy

Don't reuse `easy_install` command in `Distribution.fetch_build_egg` implementation.

Fix #196.

Fix exception on mingw built Python 2

msvc9compiler doesn't like being imported on mingw built Python. It
throws DistutilsPlatformError, so catch it.

Fixes #1118