Cleanup
TODO
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.
Fix sound resource paths

- Set pyglet resource path and reindex the files so that it can
  find and load sound files.
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.
Bugfix
Merge remote-tracking branch 'avelanarius/master'