Differential queries feature

Please see additional methods (ListDifferential<> and ListDifferential) to support differential queries.
I put a temporary Internal field (DeltaToken) to support it - it should be refactored.
I believe the code needs some refactoring:
- It can use WebApi instead of WebClient to support generic rest calls (get/put etc.).
- The util class takes static methods that accept the instance as a parameter - this is better implemented as an extension method or separate it from the class all toghether.
- Consider Automapper to map returned objects from Azure Api into Graph Api.
- Consider checking in the Unit tests project.

Cheers,
Shai