art asset and extensibility update (see discrip)

updated the inventory and slots (personal preference..), added
inventory.xcf overlay for easy changing as a template by the Gimp, and
made TERRAINMAP_BLOCK_SIZE for easy adjusting for larger terrain.png
files
add 157139a from boskee, see addit'l info please...

For some reason windows wont let me do path.join() for folders, so i
have to keep all assets in the main folder.  Any suggestions would be
appreciated.
res folder, modified from boskee

organized now, had ta friend help me to troubleshoot whey a function
worked in linux, and not in windows.  ended up had to re-index the
resources folder.  if we hack this a bit more, as boskee suggested, we
may be able to do skins and texture packs.
Improves performance.

First improvement: the queue now avoids doing opposite tasks.
Second improvement: the queue has been split in an urgent one and a lazy one (new displayed sectors=urgent).
 Third improvement: "World.is_exposed" is now cached.
Reduces cacti width and enlarges is_exposed.
Merge https://github.com/boskee/Minecraft
Fixes tree distribution.
Temporary solution to sound problems.
Merge commit '5b221b72ba7dfb0a847282d7a8b454041b30424f'
Fixed save system (saving whole World object caused to save for example batch object).
Revert "Fixed save system (saving whole World object caused to save for example batch object)."

This reverts commit 47937399bfc4e8e4adf04458353db4040d9ef0ff.
Use pyglet resource.path to index sound resource files
Thanks to PR #32 by @tfaris
Pyglet does not like backslashes at all, and wants to be fed /'s only
Show cracks while breaking blocks
Bugfix
Merge remote-tracking branch 'avelanarius/master'
Merge commit '1161ef80ccb31d9c894fe592c7cdeb5c91716c50'

Conflicts:
	sounds.py
Fixed save system.
Added leaves break sound.
Generate stone blocks beneath the surface.
Added ore generation on random depth.