Port for Windows Store apps

Hi Daniel,

I ported your library to WinRT for one of my projects and I thought, others might find it useful, too.

What I did:
- Created a new VS project for a Windows Store library
- Used the signing algorithm provided by a [Microsoft Sample] (http://code.msdn.microsoft.com/windowsapps/Web-Authentication-d0485122)
- Removed all references to NameValueCollection

The last two points were necessary since WinRT does not support all of the classes that were used in the original code. At first, I thought about reimplementing NameValueCollection, but then decided that Dictionary\<K,V\> should be sufficient.

Please have a look at it and if you like it, I would love to see it available in your Nuget package soon.

Regards,

Michael