MGO-35 add support for ServiceHostname for GSSAPI, test for ServiceName

See corresponding code in server's MongoClient [here](https://github.com/mongodb/mongo/blob/be05259d36ec5f44c821c8f2d103a4ee83ebaf53/src/mongo/client/sasl_client_session.cpp#L279-285)
Please drop the commas at the end of the line, and also run go fmt to ensure the code is properly formatted according to the usual conventions. There are spacing issues below.