Add cassandra 2.0

According to the project website (http://cassandra.apache.org)
version 2.0 is the most stable version. It's the one that's
recommended if you are already in production or going into
production soon.

Cassandra 2.0 is also the version that's officially supported
by Titan (http://thinkaurelius.github.io/titan), a distributed
graph database that uses cassandra as one of it's backends.