Changed the assetUrl function to test whether the input parameter is valid.
Clean up on PR https://github.com/code-dot-org/blockly/pull/302.
Fixed indentation.
Resize the pegman for birds skin.
Added hopping bird.
Had to slow down some level so that the hopping animation has enough time to play.
Removed array from the loop and replaced with a function.
Moved the function out of the loop.
Updated based on Brandon's comment.
Merge pull request #306 from nanan9177/maze_skin

Clean up on PR code-dot-org#302. + Adding bird hopping