Add translator for RethinkDB

RethinkDB is built to store JSON documents, and scale to multiple machines with very little effort. It has a pleasant query language that supports really useful queries like table joins and group by, and is easy to setup and learn.

See http://rethinkdb.com/.

There are two 3rdparty drivers for Java, so we should pick one and build on that:

• https://github.com/dkhenry/rethinkjava