WIP: RethinkDB for blaze

- [ ] `into`
- [x] `discover`
- [ ] `By`
- [ ] `ColumnWise`
- [x] difference between `t[[column]]` and `t[column]`? no, the way to select "columns" is with `pluck`
- [x] use `RqlQuery` everywhere except a few places where a `Connection` is needed
- [x] check out `post_compute` uses the `Connection`
The official Python RethinkDB library isn't Python 3 ready. For example, there's usage of old python 2 APIs like `iteritems` in their code. I would guess that it's much less trivial of a port than fixing all the API differences.