fixed issue that caused KeyError on stray colons
Apply @bmuller's changes from pull request #18 - move the disconnect method from TestCQLClient to CQLClient and add a disconnect method and tests to cluster.
Fix test for disconnect method in CQLClient
Update the docstring of the disconnect method.
Merge pull request #19 from bmuller/prepare_fix

fixed issue that caused KeyError on stray colons
Merge pull request #21 from rackerlabs/bmuller_connection_closing

@bmuller's disconnect from cassandra in clients and clusters
not retrying when all nodes are down

- removed clock and other retry args.
- updated tests accordingly

Yet to add test for race condition @dreid mentioned
added multiple client call test
Merge branch 'master' into auto-388-reconnection

Conflicts:
	silverberg/cluster.py
	silverberg/test/test_cluster.py