Cocoa generator improvements (service inheritance / server-side exception handling) and an implementation of the Thrift tutorial

Cocoa generator improvements relating to:
  - **Server-side exception handling** in the actual TProcessor implementation for the service
  - Better handling of **service inheritance**
    - Service clients can now use the client subclass directly to invoke calls to the base service
    - Servers must only adopt the child protocol and will automatically adopt the base protocol over inheritance
    - The improvement should not break any existing service implementations
      - See the revisions for the implemented cocoa tutorial for instance

An **implementation** for the **Thrift tutorial**:
  - Server and client implementation based on the corresponding Python version
  - Update relating to the improved handling of service inheritance