Include error in error logging.
Major refactor of node pooling strategy.

Node connectors and connections are now keyspace aware. You no longer
have to specify an explicit keyspace on connecting to a cluster, but
commands to the cluster are now bound to a particular keyspace. Node connectors
are smart about allocating connections to a given keyspace.

Additionally, fixed a couple bugs in the Session interface and also
added a bit of initial sanity checking around the existence of a keyspace
when constructing a session.

Also bumped the version to 0.2.0-SNAPSHOT due to a couple API changes.