Allow configuring `cqlVersion` in Cassandra backend

For `cassandra_db.js`, which can implicity be considered `cassandra_cql2_db.js` now, if you want to run CQL2 on Cassandra 1.2.x or higher, you'll want to be able to configure `cqlVersion: '2.0.0'`.

Coincidentally, the fact that helenus is being set to version `0.6.2` is leveraging a bug where helenus will default to CQL2 when connecting to Cassandra 1.2.x+. However, if helenus is upgraded to 0.6.3+, the driver will effectively be broken on Cassandra 1.2.x as-is. So this should be fixed before upgrading that module.