notification changes

-only send a notification when the person you follow creates a topic
-you still get a notification per post if you are following a topic
-changed notifications.push so that it sends the notifications over a
period of time, currently to 50 users per second
-optimized topics.notifyFollowers and
user.notifications.sendTopicNotification, they no longer query the
database for the topic and post data instead they get it as params
-you can no longer follow yourself :)
-changed mongo sortedSetRemove so that it doesn't use $in if there is
only a single value to remove