Add ResponseCallback class

When accepting raw HTTP response one may use the following code:
```java
@GET("/users/list")
void userList(Callback<Response> cb);
```
This results in `Callback` with two equal objects in `success` method: 
```java
service.userList(new Callback<Response>() {
      @Override
      public void success(Response response, Response response2) {
      }
      @Override
      public void failure(RetrofitError error) {
      }
});
```
By using `ResponseCallback` class code becomes a little bit cleaner:
```java
service.userList(new ResponseCallback() {
      @Override
      public void success(Response response) {
      }
      @Override
      public void failure(RetrofitError error) {
      }
});
```

I'm not sure if this class is needed in retrofit. But I'm using it in my projects so I've decided to propose this.