Support for RethinkDB 1.0 handshake protocol

[tuolsleng]

RethinkDB 2.3.5+ databases use a new handshake protocol that allows to connect using another username than "admin". That impacts the connect function (which currently only has a Maybe String parameter for the password only). I had a look at the official ruby API implementation and, sadly, I lack both time and knowledge to patch the package to support the RFC 5802 handshake :(