Labyrinth Entrance Type

I implemented a Labyrinth entrance type.  It's basically a maze generated using the caver algorithm that surrounds the entrance shaft.  It's one chunk wide and long.  The walls near the middle of the maze are higher than the walls on the edges of the maze.  There is one entrance to the labyrinth on each of its four sides.  Also, I chose not to hollow out the maze with Air, meaning that it's possible for tree leaves and such to be present inside the labyrinth.  I think it adds to the "run-down" atmosphere.

I didn't add it to any of the config files besides default.cfg because I wasn't sure what the probabilities for it should be, so I thought you guys should make that decision.

One last thing... I added the "Ice Plains" biome to the _ice_biomes tuple at the top of ruins.py.

Here's some screenshots:
![2013-11-07_20 59 02](https://f.cloud.github.com/assets/2229288/1498177/bbbce328-4826-11e3-95f7-6785ba749098.png)
![2013-11-07_20 58 44](https://f.cloud.github.com/assets/2229288/1498178/bec5cfb2-4826-11e3-9ba9-48cd3c9f8925.png)
![2013-11-07_21 01 17](https://f.cloud.github.com/assets/2229288/1498183/cf5560a4-4826-11e3-871c-8256113dba78.png)
![2013-11-07_20 59 15](https://f.cloud.github.com/assets/2229288/1498185/d52d0946-4826-11e3-9d29-52cc1e2cfd90.png)