WinRT Support

Builds upon #1 

TODO:

 - [ ] 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?
 - [ ] Update nuspec to pull in build output from WinRT build
 - [ ] Q: do you have a preferred test framework? (please say xUnit. please say xUnit).
 - [ ] Incorporate test project for WinRT extensions as starting point
 - [ ] Test the new extension methods behave as expected