Suggested improvements in the Cassandra sample

This Pull Request brings some suggested changes to the Cassandra sample:
- Uses PreparedStatements instead of concatenated strings
- Replaces AlreadyExistsException with "CREATE TABLE / KEYSPACE IF NOT EXISTS" statements that have been introduced in Cassandra 2.0
- It also changes the replication factor to 1 in the keyspace definition so that there won't be any issue if used with a single node cluster