#335 Add tests for Windows 8.1

As suggested in #335. This does seem to just work on the couple of machines that I have tested it on, which were running Windows 8.1 and Visual Studio 2013. The project builds and the test passes.

The [standard upgrade process](http://msdn.microsoft.com/en-GB/library/windows/apps/dn263114.aspx) also made changes to the minimum Visual Studio version (to require VS2013) but I don't think that this will be necessary, everything should also run in VS2012. However I do have those changes in a separate [commit](https://github.com/jamiehumphries/FakeItEasy/commit/637f2e5e94a937232923f96072680eff8704dfe5) that I could add to this PR if you are happy to require VS2013.