Prototype Implementation of API 2.0

We have started on a prototype implementation of API 2.0. Because API 2.0 is still under development, we have avoided modifying the Retrofit internals as much as possible. Instead, the Call<T> class uses the existing RestHandler internally to implement execute() synchronously and asynchronously. We have avoided naming conflicts for the Callback class by calling it Callback2 (for 2.0).

The idea here is twofold: One, the API can be refined and developed further with an actual implementation to experiment with, even though the internals are not fully developed. Second, if the API 2.0 prototype is further fleshed out tests and future clients can start porting to API 2.0 while the internals are developed.

This is a very simple starter implementation, and does not support cancel() or retry() or the error handling functions, because they are non-trivial to implement without significant modification of the RestAdapter and RestHandler. Therefore, I would not recommend using this for actual client code but you may find it useful for prototyping.

edit: I should point out that this was implemented alongside the existing code, so it shouldn't break any existing clients or tests.