Adds a MongoDB broker

* adds mongo broker tests
* adds mongo brokers docs

* broker info is now cached when it needs to connect to a server, to reduce traffic.

django_q/tests/test_cluster.py

@@ -33,6 +33,7 @@ def broker():
     Conf.IRON_MQ = None
     Conf.SQS = None
     Conf.ORM = None
+    Conf.MONGO = None
     Conf.DJANGO_REDIS = 'default'
     return get_broker()