First cut at a gevent connection class.

It seems to kind of work. The integration tests aren't working
right; that needs to be fixed before first. A simple test of the
basic functionality seems to work, but I've not yet tested the
recovery/retry and other execptional conditions.
Remove unused set.
Clean up.
Clean-up and refactor.
Use gevent as default connection class when monkey patched.
Bring back the timeout.
Small cleanup.
Add version field, setter to Host
Add protocol_version attr to Cluster, use in conns
Add dict support for bound statements
Better logging for dict KeyErrors
Missing period
Fix whitespace
Merge branch 'master' into 2.0
Add SERIAL, LOCAL_SERIAL ConsistencyLevel enums
Check proto version for messages, raise UnsupportedOperation
Handle OrderedDict import attempt in cassandra.util
Move encoders from decoder.py to encoder.py
Move row factories from c.decoder to c.query
Define BatchType enum