Implement Tree Api Client

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