Rename image to sprite
add tree chop sound
add placeholder lumber image
Lumber sprite spawning

-spawn a lumber sprite each time we remove lumber from the tree.  It
spawns a new sprite and starts a tween to move it from the position of
the tree to the GUI element.

This sort of works, but for some reason multiple pieces of lumber stack
up and things break down.
Merge branch 'master' of https://github.com/Levo/Pixi-Js

Conflicts:
Paul/js/tree.js
?
Resolve conflict?