Abstraction for BATCH statements and executing multiple prepared statements at the same time

Currently, the client doesn't include any abstraction for using `BATCH` queries or executing multiple prepared statements at once. 

JDBC exposes `preparedStatement.addBatch()` and `preparedStatement.exesBatch()` methods. This client seems to follow JDBC API and Cassandra recently added support for this (https://issues.apache.org/jira/browse/CASSANDRA-4693) so adding a similar methods to this client seems reasonable.

I've also included a very naive approach in this pull request. This approach has multiple disadvantages like requiring a user to send a tons of strings over the wire so I don't see it being merged. I've pretty much only included it to get a discussion rolling :)

Feedback and comments are welcome.