Fail fast when gevent monkey-patching is detected

Fixes #43

cassandra/cluster.py

@@ -5,6 +5,7 @@ This module houses the main classes you will interact with,
 
 from concurrent.futures import ThreadPoolExecutor
 import logging
+import sys
 import time
 from threading import Lock, RLock, Thread, Event
 import Queue
@@ -213,6 +214,12 @@ class Cluster(object):
         Any of the mutable Cluster attributes may be set as keyword arguments
         to the constructor.
         """
+        if 'gevent.monkey' in sys.modules:
+            raise Exception(
+                "gevent monkey-patching detected. This driver does not currently "
+                "support gevent, and monkey patching will break the driver "
+                "completely. You can track progress towards adding gevent "
+                "support here: https://datastax-oss.atlassian.net/browse/PYTHON-7.")
 
         self.contact_points = contact_points
         self.port = port