Added PushMulti, a GroupFunction to do a push with multiple fields.
Update facade to allow the ActorSystem as a parameter.
MongoDriver.close() gracefully terminates actors that are created in a non default ActorSystem.
Merge pull request #73 from craigtmc/master

Add the ability to do a push with multiple fields.
Merge pull request #74 from spryly/master

ActorSystem as a parameter to MongoDriver