Adds Xenobiologist as its own job.

Relevant thread:
http://baystation12.net/forums/viewtopic.php?f=5&t=7751

New job prefs panel looks like this:
![xenobio3](https://f.cloud.github.com/assets/1355638/639737/3fd53b22-d2db-11e2-840b-2fb357495ad5.png)

(Just checked my map and there might be a conflict, not sure though but it looked fishy)