Project added for building MonoGame apps for the Windows8 Store

I have created a (VS2012) project for a Windows Store Library for use with MonoGame.  There are no new code files because the project will simply link to all the source files in the spine-csharp and spine-xna folders.  Therefore any changes to these source files will be automatically picked up by this project.  Any new source files for future runtime features will have to be added to the project however this can be done as and when required via a pull request.

I thought this would be useful for MonoGame developers as they won't have to create the project files themselves.  

If you are happy with this approach then I'll also send a PR for a XamarinStudio project file for iOS MonoGame developers.