Adding batch file creator for Windows

Hi,

I installed mkdocs via pip on Windows 7, and found that I couldn't run it from the command line.  Turns out it wasn't creating a batch from which it could run, so I copied and amended an [example](https://github.com/waylan/Python-Markdown/), which seems to run fine locally using `python setup.py install`.

I wasn't sure how to contribute to this project, hence just opening this pull request.  I couldn't see any outstanding issues, and the [wiki](https://github.com/tomchristie/mkdocs/wiki) mentions Windows installer support, so hope this helps!

Thanks,
Darren