Upgrades Cassandra to 2.1

Hi,

This is a pull request to help people struggling with Guava version problems.

I've made it for my personal use, but it seems that it might benefit others, this is the reason I'm creating this pull request.

As explained in #84 The only way to solve the Guava version problem is to use Cassandra 2.1, it ships with version 16.0 of Guava which is the version where the api changed.

Migrating to Cassandra 2.1 have two major issues:
* Java version must be 7 (this might not please users of the library, but JDK 7 reaches EOL early next year)
* I Updated The CQL driver to 2.1.1 (it has support for latest Cassandra goodness)

It wouldn't surprise me if this PR was too far fetched, or maybe too early, but I think it might be good for users.

All in all, it could be possible to manage two versions of CassandraUnit for two versions of Cassandra...