Added mkdocs.bat creation at install-time on windows environments.

This allows windows user to get command-line mkdocs access out-of-the-box.
I basically [re-used what Python-Markdown was doing in their own setup.py (around line 43.)](https://github.com/waylan/Python-Markdown/blob/master/setup.py)

Only difference from Python-Markdown is on line 76 (`self.get_outputs().append(bat_path)`). It's there so the .bat file gets deleted when you uninstall mkdocs.