Changed Oil Generation to use config file ID's for generation 

This is to support integration with mods like Biomes O' Plenty so you can customize which biomes are more likely to generate oil based on the ID. Someone should probably look into making the defaults a little better as it's not done by type and it could/should be at least using that as well.

Also consider changing the default id's of biomOilDesert and biomeOilOcean so they don't conflict with Biomes O' Plenty.