Prelimiary consistency support

I couldn't figure out how to pass in a desired consistency using Database.execute, but it doesn't look like compression is supported that way either, so maybe that's not a big deal.  Calling Statement.execute or Database.execute_cql_statement will support it though.  It also should be fully backwards-compatible with the old method sigs.

Also, I had to run thrift-client 0.9.  I can't remember if that was a Ruby 2 issue or a Cassandra 1.2 issue.  At any rate, I've been using it for a while now and, at least for running CQL3 statements, it seems happy enough.  You can probably ignore that part of the commit though if you're not ready to trust it yet.  It should still work in current environments.