Add using retrofit.Callback and retrofit.ResponseCallback

Add using retrofit.Callback and retrofit.ResponseCallback in service methods with setting ```useRetrofitCallback``` boolean field of ```ServiceMethod``` closure.
This is an entity that should not know anything about Retrofit. This entity is a part of the specification DSL.
Instead, this flag should be a part of [retrofit generator options](https://github.com/stanfy/helium/blob/master/helium/src/main/groovy/com/stanfy/helium/handler/codegen/java/retrofit/RetrofitGeneratorOptions.java), which can be used with gradle plugin or command line tools.