Prevent use keyspace

As per the discussion on the mailing list (https://groups.google.com/forum/#!topic/gocql/IS3YCbiIP-A).

This code now returns an error when a user tries to execute a use statement query. I also added a test case to make sure the correct error is returned.

I have updated the README.md file with the important change and documentation of how default keyspaces must be defined.

I also cleaned up the code used to update the connection pool keyspace when a user would execute a use statement.

The only issue I see at the moment is a large amount of reconnects when the keyspace is incorrect while the session is starting up. I am not quite sure the best way to handle this in this patch but the reconnection code is consistent with the old change keyspace code so I don't see it as a new issue being introduced.