Switch from erlcql to cqerl.

Unfortunately, erlcql only supports CQL3 over Cassandra binary protocol
v1, while for recent Cassandras, we want v2. Thankfully, cqerl supports
v2, and converting the cassandra_cql driver is reasonably simple.

This patch does just that - converts from erlcql to cqerl.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #150 from algernon/f/erlcql-to-cqerl

Switch from erlcql to cqerl.