Query can be cancelled

Cancelling a running query will try to disconnect TCP connection
Option to disconnect on cancellation of running query

Done in client.py.
reverted cancellation in thirftclient

since cancellation of cqlclient will call its disconnect which will call
thrift client's disconnect
updated readme and version
ensure val before cancellation is correct - thanks @cyli
RoundRobinCluster also takes disconnect_on_cancel option

to use it when constructing CQLClient
Merge pull request #44 from rackerlabs/cancel

Query cancellation can disconnect TCP connection