Allow querying on a secondary index

[nickmbailey]

My initial thoughts here would be to allow additional options to be passed to the already existing 'get-rows' function. Since the mongodb api seems to be often referenced as extremly developer friendly maybe something similar to a clojure lib they have?

https://github.com/aboekhoff/congomongo

Basically a ':where' option followed by a clause. Will have to be some cleverness with serializing the column name specified in the clause. I think it needs to end up as a byte array before being sent to cassandra.