Sulphur ore generation

There's a lot of tweaking I'd like to do, but I think this is a pretty good result for a first try.

I tried using WorldGenMinable to generate the clusters of ore, but it had a habit of spawning 6-7 blocks away from the lava.

I'm not quite game enough to put in the configuration options myself yet as I'm still trying to get a feel for your coding style before I try doing infrastructural. I have however declared the configurable settings as variables at the top of the method.
We need to incorporate ````Block.isReplaceableOreGen()```` in here somewhere for compatibility with mods that add custom stone and dirt.