Adding support for Cassandra CQL3

Adding a CQL3 driver that is backward compatible with cassandra_db.js (CQL2)
by using a compact storage table.

I have named this driver `cassandra_cql3_compact.js` in anticipation that the
"defacto" CQL3 driver would use the newer non-compact CQL3 storage model,
and would likely want to `cassandra_cql3.js`.

Also added a test to get coverage on the `remove` function. Note the
documentation on how to upgrade from `cassandra` to `cassandra_cql3_compact`,
as there is a necessary schema statement for it to work.

Migration was validated manually using etherpad as the ueberDB consumer.