Update project for VS 2013

No functional change. Update projects to also open in Visual Studio 2013.
Mark NonNullable with DataContract

- PCL doesn't support [Serializable] so mark NonNullable with
  [DataContract] so remoting / serialization is possible
- Add matching tests
Add DataContract to Exceptional

- PCL doesn't support [Serializable] so marking Exceptional with
  [DataContract] so some remoting / serialization is possible
- Base Exception type isn't marked as [Serializable], so changed signature
  of Exceptional.Exception to be generic since most concrete types are
  serialiazable
- Add matching tests
- Update README