CAS support for Hector

I finally got time to work on CAS support and finished it. There's 1 unit test for it in MutatorTest and it passes. A couple of general thoughts about CAS. They way it was added to Cassandra feels like a "bolt-on", as a result it does feel a bit "out of place" with the existing Mutator interface. Basically it feels like a "bolt-on" since that's basically what I did to minimize the impact of adding support.