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`
Beware that as of https://github.com/ContinuumIO/blaze/pull/481 I've expanded `By`'s apply field to also possibly be a `Summary` object.  This complicates things a bit.  I've been thinking about how to handle this added complexity.