Added check to prevent user from executing a use statement from an active session.
Updated README.md with examples of defining the default keyspace correctly for a session.
Cleaned up code to streamline the connection being added to the pool and removed code related to old keyspace behaviour.
Merge pull request #144 from phillipCouto/prevent_use_keyspace

Prevent use keyspace
Use better error messages instead of ErrUnavailable

When a session is closed it should not be possible to execute queries,
the error message for this is distinct to a live session which has
not connections.

If a live session has no available connections then indicate that
instead of the generic ErrUnavailable.
Use defer to unlock in session.go