Add back Windows 8 support

This PR adds back support for Windows 8 as it still has twice as much marketshare as Windows 8.1 according to Net Applications (http://thenextweb.com/insider/2014/01/01/windows-8-windows-8-1-pass-10-market-share-windows-7-still-gains-windows-xp-falls-30/#!r1NYr). Also we currently do not use Windows 8.1 specific API's.

I moved the Windows 8.1 assemblies into the WinRT451 folder to make sure they do not get added to a Windows 8 project, because that crashes at runtime.
I also added a WIndows 8 specific test project in addition to the Windows 8.1 project.