massive cassandra tidy-up:
 * makes CassandraFabric work even better, giving control over token allocation, and ensuring (not just preferring) one seed from each datacenter
 * add an addChildMember to Group (as CassFabric requires the datacenters to be *members*)
 * rename CassandraCluster to CassandraDatacenter to fix the ambiguous name
 * misc changes to Cluster (Datacenter) and Node to make it easier to consume, run scripts, and subclass
add back CassandraCluster as deprecated subclasses, to preserve compatibility