Added Tribbles.

They reproduce every 30 Life() ticks or so, until their global numbers are 50.
They can be picked up and turn into an item upon doing so, when this item is attack_self'd the tribble is placed as a mob again on the floor.
Tribbles can also be neutered with a scalpel to stop them from reproducing, but they must be held as an item or else they "move too much" and you kill them. Neutered tribbles may also be "un-neutered" with a cautery.

--Adrinus

Some pics to illustrate:
Before:
![e85a0e479ca5ef9722d4f8d0f969c097](https://f.cloud.github.com/assets/2319040/1598208/736ee71c-5328-11e3-8069-4c963c3c70d0.png)

Some Minutes Later:
![6a0e070ecdaf1d14216a3deba092c374](https://f.cloud.github.com/assets/2319040/1598209/827c66f8-5328-11e3-9286-9780073c84c2.png)

A Solution:
![7b32a07f80671a18fe581367da3b5fe7](https://f.cloud.github.com/assets/2319040/1598212/8b963566-5328-11e3-99cf-507c0f80083f.png)