16 layers deep ground, random ores and gravel

extended the ground to -16, made terrain generation use a system of
common ore blocks (gravel, stone) along with random ores.  you will get
more common blocks than ores though.
Merge commit 'c9478c2fb06d9c732498a98c18c3add2ced639ec'

Conflicts:
	main.py
	resources/textures/texture.png