Fork retrofits way of creating request methods.

[serj-lotutovici]

Currently each resource declares a method, that returns a CallSpec, each declared method uses helper factory methods to build the resulting spec. This created multiple problems with the api flexibility, and is is not type safe. A good approach to adopt the reflection based method, to provide an implementation for each method like Retrofit does, but in a more restricted fashion:

• Allow returning only CallSpec
• Support only GET, PUT, POST and DELETE requests