Cassandra prototype

@6a68 - Yo, I added support to run the backend tests against Cassandra, though they stall out.

```bash
➜  ~PERSONA_ROOT git:(cassandra_prototype) ✗ WHAT_TESTS=back_cassandra npm test

> browserid@1.0.0-b2 test /Users/stomlinson/development/browserid
> ./scripts/test

>>> Now Running: API level unit tests using Cassandra
lib::db:cassandra.open intercepted. Args: {
 "0": {
  "driver": "cassandra",
  "may_write": true,
  "max_query_time_ms": 5000,
  "max_reconnect_attempts": 1,
  "cassandra_keyspace_properties": "{'class': 'SimpleStrategy', 'replication_factor': 1}",
  "cassandra_hosts": [
   "localhost"
  ],
  "cassandra_cql_version": "3.0.0"
 }
}
lib::db:cassandra.close intercepted. Args: {}
tests/account-cancel-test.js
✗lib::db:cassandra.open intercepted. Args: {
 "0": {
  "driver": "cassandra",
  "create_schema": true,
  "may_write": true,
  "max_query_time_ms": 5000,
  "max_reconnect_attempts": 1,
  "cassandra_keyspace_properties": "{'class': 'SimpleStrategy', 'replication_factor': 1}",
  "cassandra_hosts": [
   "localhost"
  ],
  "cassandra_cql_version": "3.0.0"
 }
}
·
```