WIP: RethinkDB for blaze

`post_compute` was designed to handle this sort of thing.  It's called on the final result after all of the `compute_one` calls are made.  It's not a perfect abstraction but it's managed so far with other systems with similar configurations like `sqlalchemy` and `mongodb`. 

See:

https://github.com/ContinuumIO/blaze/blob/master/blaze/sql.py#L18

https://github.com/ContinuumIO/blaze/blob/master/blaze/compute/mongo.py#L220

Although I guess in your case we're not sure where the connection comes from.  Is there an object that contains both the rt Table and the connection?