Fix "mkdocs" command creation in Windows

This makes a "mkdocs" command on Windows/OSX/Linux without any platform-specific code in setup.py.

I think this is the preferred way to install a Python command with setuptools.

It is, at least, the method used by (among others):

* [flake8](https://bitbucket.org/tarek/flake8/src/8ee94d1eeea36e835583b68c59682bfc3684fab9/setup.py?at=default)
* [coverage.py](https://bitbucket.org/ned/coveragepy/src/ca875e739048c3b333e862471b89d55e3c7b7170/setup.py?at=default)
* [Fabric](https://github.com/fabric/fabric/blob/master/setup.py)

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I finally figured out what was causing my previous windows-related PRs to get all funny about imports:

In the past, I had to relativize imports willy-nilly or add `from __future__ import absolute_imports` to a bunch of files.
This is because I was renaming `mkdocs/mkdocs` to `mkdocs/mkdocs.py`, and the module-vs-script name clash was confusing imports from other files.  Now I'm changing it to `mkdocs/main.py`.