Use RESTMock for mocking Retrofit REST services

Regarding #114, we have been using [RESTMock](https://github.com/andrzejchm/RESTMock) to mock Retrofit calls and it's working like a charm! It uses [OkHttp's MockWebServer](https://github.com/square/okhttp/tree/master/mockwebserver) and it allows to define mock responses for matchers. For instance:

``` java
RESTMockServer.whenGET(pathContains("users/defunkt"))
.thenReturnFile(200, "users/defunkt.json");
```

It also permits to verify if the calls have been done.