Enable AppVeyor Windows CI testing.

This PR adds the files needed to use [AppVeyor](https://ci.appveyor.com) to do CI testing of Astropy under windows (Server 2012 R2). The build matrix is smaller than your travis one but also uses miniconda, and the mingw compiler available through conda on windows.

I have enabled Python 2.6, 2.7 and 3.4 on Numpy 1.8.1 and 1.7.1 (not on 3.4).

I switched it on for my fork, and the first builds can be seen here: https://ci.appveyor.com/project/Cadair/astropy.

AppVeyor integrates into GH in the same way Travis does, however the API on GH's side is not smart enough, so the build status only shows for the last build to report.
APPVEYOR needs to be in quotes. Also, you can remove the old 'platform' import here