even more terrain options

-terrain uses string templates now. -terrain (plains, mountains, desert,
island). Trying to get the stone mountains covered in grass, failing
horribly, so not implemented.
commits boskee -avelanarius and terrain opts

-terrain [plains/mountains/island/desert]  if not specified, defaults to
normal generation
Update README.md
snow terrain

-terrain snow and snowblocks added
commit 55d3f053f4aa4c6864c98a3fa395b2ddb4119450

boksee Simple Light Source addition
Merge commit 'db4909223ab737fb675540e2331fe38c2e334934'

Conflicts:
	README.md
	blocks.py
	inventory.py
	main.py