support lz4 compression if it is available

Use https://github.com/steeve/python-lz4 to provide lz4 compression support.
I couldn't verify that OptionsMessage() will actually return 'lz4' as a supported COMPRESSION option...

UPDATE: should have checked the source more closely, lz4 is not currently supported as a native client protocol compression option, so this fix will need: https://issues.apache.org/jira/browse/CASSANDRA-5765 in order to be usable.