adding two new methods of getting replication delay as a trigger to pt-stalk

* I rewrote slave delay on Seconds_behind based on single line of awk
* If your env uses pt-heartbeat (one of mine does, with interval=0.1) then trg_slave_delay_heartbeat is what you need.  Notice you need to hard-code server_id which isn't great if you have a lot of master failovers