Reimplement Response#{value, value=} using response class.  #3410.
Eager-load a response's questions and answers by default.  #3410.

Response#value and Response#value= will always load the answer, but on a
philosophical point, I don't think a Response makes sense unless you
have its answer and question contexts.  Might as well just load them up
front.
Merge pull request #55 from NUBIC/task-3410

Reimplement Response#{value, value=} using response class