trace algoritm

I'm actually kinda surprised by how this worked well. It's just a small modification of the A star finder giving a greater priority to nodes with few neighbors. It does perform better than A* in lots of cases and behaves a lot like an actual trace.
![astar](https://cloud.githubusercontent.com/assets/422159/3165913/b2bdf64a-eb5a-11e3-985e-9c2954b82f92.png)
![trace](https://cloud.githubusercontent.com/assets/422159/3165930/dd1606e4-eb5a-11e3-8335-a77450b458d4.png)