* `Call` is the mechanism for request execution. It encapsulates a single request action either synchronously or asynchronously, and can support canceling.
* `CallAdapter` is a means of supporting a method return type other than `Call` (like RxJava's `Observable`) by adapting the `Call` instance into another instance. This change moves RxJava support into a sibling module which uses an adapter for supporting `Observable` types.
* `Response` represents the HTTP response along side the deserialized body. It allows access to both the converted body and lower-level items such as the header, response code, and underlying OkHttp request and response objects.

(cherry picked from commit 62903c4b)

(cherry picked from commit 2f4caa6f)

(cherry picked from commit 3b1df8be)

Conflicts:
	pom.xml
	retrofit-converters/jackson/pom.xml
	retrofit-converters/pom.xml
	retrofit-converters/protobuf/pom.xml
	retrofit-converters/simplexml/pom.xml
	retrofit-converters/wire/pom.xml
	retrofit-mock/pom.xml
	retrofit/pom.xml
	samples/github-client/pom.xml
	samples/mock-github-client/pom.xml
	samples/pom.xml