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
set key as well

not sure if this fixes dupe keys
display user count in browsing
removed this wasn't the problem after all
get correct count only from one of the active users elements
reverting #2259 temporarily until we figure out why logs aren't showing up
Revert "reverting #2259 temporarily until we figure out why logs aren't showing up"

This reverts commit 59b3e6f78fdcdb4dfbded9d65dfd58e08c0a6355.
updating markdown minimum version to 0.7.x and up
optimize getUnreadByField

only get the most recent 100 notifications
dont call UserNotifications.getNotifications which does alot more work,
call the db directly
Merge branch 'master' of https://github.com/NodeBB/NodeBB
added nid
cleanup and optimize notifications
user onlineUsers instead of calculating from rooms
emit stats to registered users only
fix notifications to actually get recent
dont search empty string, get 20 tags instead of 10
closes #2278
#2278
fix browsing users removal
closes #2276