Added support for Cassandra.

This PR adds support for the [Cassandra Database](http://cassandra.apache.org/). It assumes that the keyspace you're working in exists. The columnfamily doesn't have to be present and can be created on initialization.

```bash
$ node benchmark.js cassandra
<..>
rows: 99800
finished
time: 401s
```
Disabling findKeys and removing all logic to support it shaves about 40 seconds of off the benchmark.

Let me know if something is missing, incomplete, incorrect or not according to a styleguide.