WinRT Support

TODO:

 - [x] Q: I've added a csproj into the same folder rather than doing a separate folder and linked files - what would you prefer to support? A: Multiple csproj files in the one folder it is!
 - [x] Update nuspec to pull in build output from WinRT build
 - [x] Q: do you have a preferred test framework? (please say xUnit. please say xUnit). A: xUnit!
 - [x] Setup test project for WinRT extensions as starting point
 - [x] Test the new extension methods behave as expected