Implement Tree Api Client

Ready for review.  Fixes #132

- [x] Create TreeClient
    - Implement Get
    - Implement Create
    - Create Interface
- [x] Create Tree response model
- [x] Test Tree Client
     - Test Get
     - Test Create
     - Test Serialization
- [x] Create ObservableTreeClient
     - Create Interface
     - Test ObservableTreeClient
- [x] Reference in GitHubClient
- [x] Add files to other projects
     - .Net 4.5
     - Mono
     - MonoAndroid
     - MonoTouch