Change Character class to GamePiece to abstract as superclass for all items, not just characters.
Add gem sprites to cache in engine.js
Add functions to instantiate a gem.
Fill gem properties from random gemtype
Update gem to automatically reset after this.duration seconds
Make gems appear and disappear randomly on the board using placeholder class to replace
Update placeholder to take a callback function so that replace gems will have destroy time properly set at instantiation.
Update comments in app.js
Refactor destroytime in Gem to use fewer variables
Remove gem from board when player collides.