Cassandra 1.2 changes

This branch includes the following changes:

- Includes new generated Thrift files which have been generated using Cassandra 1.2 Thrift definition files. This allows client to also work with Cassandra 1.2 using Thrift transport.
- Allows users to specify which CQL version is used by passing `cql_version` attribute to the `Connection` and `PooledConnection` class constructor.

With this changes (and setting `cql_version` to `2.0.0`), existing code which works with 1.1 should also work with 1.2.

Keep in mind that this branch doesn't implement new native binary protocol. This will be done in a separate PR at some later point (contributions are welcome).

Note: For some reason Travis build doesn't pass. I'm not sure why because they claim they use Cassandra 1.1 and I've tested it locally and tests pass with both 1.1 and 1.2.