Make retrieving a user's posts easier

I had some trouble converting map[string]interface{}s to different objects, so I went for the less generic route and:

- changed ResponseEnvelope to ObjectResponseEnvelope (Data is is of type interface{})
- added ListResponseEnvelope (Data is is of type []interface{})
- added PostsResponseEnvelope (Data is of type []Post)
- added metaResponseEnvelope (Data is nonexistent)

There're likely more to come in the future; AFAICT if I don't do this, I'll be making oodles of type assertions in client code instead of having json.Unmarshal nicely fill in all the fields for me.

metaResponseEnvelope is for testing for the presence of an error right there inside of adn.Do(). While I'd like to be able to simply unmarshal into `v` and read things out of there, I can't be sure yet that `v` will even have a .Meta field, and I'm not sure I want to enforce that constraint yet.