Added BiomeSpecificOreGenEvent 

As of now, this event is simply used for Emerald generation in reference to #1158. I wasn't exactly sure if this is what was intended by Lex's last comment of "need to find a different event to figure it," so this is what  I quickly put together. If this isn't what you meant or if you wanted to reuse the GenerateMineable event (or obviously, if you wanted this implemented in a different way) just let me know. I think I kept the patch as minimalistic as possible, but if you see a way that I could reduce it further, again, just let me know. 

A simple example mod utilizing this event can be found here: http://pastebin.com/xgRzH8p4
Would `net.minecraftforge.event.terraingen.TerrainGen.generateBiomeSpecificOre(p_76728_1_, p_76728_2_, p_76728_3_, p_76728_4_, Blocks.field_150412_bA).getResult()` work?  Also, you're deleting a newline on line 7 and adding a newline on line 23.