Cassandra entity and initial Cassandra Cluster entity

Cassandra entity initial changes. Created `CassandraServer` and `CassandraCluster` entities with basic startup and shutdown tests. Only _SERVICE_UP_ and _TOKEN_ sensors provided. The cluster entity is an extension of `DynamicCluster` and has an extra _UPDATE_ effector, triggered by changes to membership, which resets the ring token of each member to spread the load efficiently.

Still needs:
- Better testing for cluster entity
- More sensors
- Test using the [Astyanax](https://github.com/Netflix/astyanax) client