Fix calls to undefined ItemStack.getItemName method.

UndergroundBIomes.java was attempting to call a method "getItemName" on instances of ItemStack. This method does not exist. I assume it was changed in the recent version of forge, and have replaced all instances of this with a  call to "getDisplayName" instead.
Merge pull request #12 from Chekote/getItemName_fix

Fix calls to undefined ItemStack.getItemName method.