Add support for pool size

This pull requests adds the option to create more than one connection per host in a pool.  

What test runnner are you using? I tested it in a few of my applications that rely on node-cassandra-cql and it seems to work great.  I was not able to determine which test runner the project was designed for.  If you tell me, then I can ensure this does not break any tests (and write one for the connection pool counts).

Great library.  Thanks for maintaining it.