Adds treaded syndie borg sprite

Named 'treadhound' in the file

Credit to Mechoid
![wov6zoa](https://cloud.githubusercontent.com/assets/7247232/2982075/cb88d55a-dc0f-11e3-8368-54df57bb1d64.png)

This is for an upcoming ~thing~ that will be pr'd later but might as well have them in now for admin sprite swaps if people wanna be tankborg