split Git for Windows and msysGit into two pages

We put the download link for git for windows on the main page and the
link to the development environment to a second page. This way people
should not get them both confused that often anymore.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>

This can solve issue #6.

You can have a look at a preview here:
http://htmlpreview.github.io/?http://github.com/msysgit/msysgit.github.com/blob/hv/split-msysgit/index.html