ratelimit: A module that rejects commands in excess of limit per interval seconds

Defaults to 60 per 60sec, ie. an average of 1 msg/second
Add extra functionality to ratelimit to kill a connection after too many messages over limit
ratelimit: Don't prevent PING/PONGs even after limit is hit

Ignoring these is a great way for clients to get timed out accidentially.
Make ratelimit a core module
ratelimit: Change default kill limit
Merge pull request #46 from ekimekim/ratelimit

Add ratelimit module