don't throw away a host that is in the contact points list
add WhiteListRoundRobinPolicy
Merge pull request #63 from devdazed/master

Add WhiteListRoundRobinPolicy and attempt to connect to all contact points
Handle setting same keyspace twice

Before this fix, the second "USE system" query would hang until an
OperationTimedOut exception was raised.

Fixes PYTHON-38
Add support for ConsistencyLevel LOCAL_ONE

Fixes #65