Slime update

Ports over this update for the slimes: https://github.com/tgstation/-tg-station/pull/3615

Forum thread with details here: http://baystation12.net/forums/viewtopic.php?f=5&t=10824

![slimesmile](https://cloud.githubusercontent.com/assets/6681708/2851821/fdca231c-d123-11e3-9299-16f0273db19d.png)

Fixes #5549