Apache Gora - Implement RethinkDB datastore module

RethinkDB is a popular implementation for document store which is being used extensively in modern real time applications. Detailed technical comparison between RethinkDB and MongoDB as a document persistent store solution is available on reference. [1] There are a number of benefits of RethinkDB as it developed focusing on developer oriented and operations oriented mindset. [2] Apache Gora currently supports MongoDB and CouchDB as document stores, this project is aimed to further extend the datastore support for RethinkDB. [1] https://rethinkdb.com/docs/comparison-tables/ [2] https://rethinkdb.com/blog/mongodb-biased-comparison/