Allow message send bursting
Allow message send bursting

Since Slack allows short burts of messages, I added bursting
support with a cooldown. The result is the robot types a lot
faster now.

uva-its/gopherbot,uva-its/gopherbot,parsley42/gopherbot,uva-its/gopherbot,uva-its/gopherbot,parsley42/gopherbot,parsley42/gopherbot,parsley42/gopherbot