Windows: Don't use os.path.join() for URL paths.
Windows: Use posixpath in nav.FC.make_absolute()

nav.FileContext.make_absolute() is only used to compare
links to entries in mkdocs.yml, so it makes sense for it
to only use POSIX-style paths.

Without this, the site fails to build because it checks for
a Windows-style path in a list of POSIX paths in
build.PathToURL.__call__().

This is the last change required to build the mkdocs site
on Windows.
Updated setup.py to use entry_points['console_scripts'] instead of scripts.

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:

* [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)
* Many others

In the past, I had to change imports willy-nilly or add
"from __future__ import absolute_imports" to a bunch of files.
This is because I was renaming "mkdocs" to "mkdocs.py" instead of "main.py",
and the module-vs-script name clash was confusing imports from other files.
flake8 fixes in main.py (How did this ever pass?)
Update the menu/toc to support the new nav code changes.

fixes gh-110
Merge pull request #122 from edbrannin/windows-command

Fix "mkdocs" command creation in Windows
Merge pull request #124 from ryanneufeld/fix-gh-110

Update the menu/toc to support the new nav code changes.
Merge pull request #121 from edbrannin/windows-paths

Fix path-handling on Windows