Add node_stone to mgv5 biomes. Remove unnecessary underwater grass hack

![screenshot_72794710](https://cloud.githubusercontent.com/assets/3686677/5242546/437b2d82-792d-11e4-9729-7480cf62dfd1.png)

^ Here i am at y = 47

node_stone = "default:xxxxxxx"
can be added to a biome definition, the chosen stone will extend down to y = -32.

![screenshot_63343680](https://cloud.githubusercontent.com/assets/3686677/5242601/2006ef70-792e-11e4-819f-5eb48c8e973b.png)

^ Obsidian biome