Distinct Tables Values: 90%

This PR allows the ability to do distinct queries on table columns
Hmm, although looking at this a bit closer, there's always been an expectation the developer to supply "value." for filters in getTableRows() (and that would still have to be there).

See:
https://github.com/talis/tripod-php/blob/master/test/unit/mongo/MongoTripodTablesTest.php#L328