Merge pull request #15 from jakerose27/master

Checkpoint and holdbuttons.
Merge pull request #16 from jakerose27/master

Checkpoint class and new image/animation.
Fixes collision detection and button logic.
Merges with Jake's checkpoint changes.
Fixes balancing issues for level 1.
Depth property added to sprites, items list being sorted for depth.
Merge branch 'master' of https://github.com/rliebz/Puddle