Refactored loader, and created new function 'removeAssets'

Refactored ```Crafty.load```, making it recieve an object with assets definitions, including audio, images and sprites.
Also, created new function ```Crafty.removeAssets```, which receives an object similar to the one that's passed to ```Crafty.load```, or even the exact same object, in which case it will delete all assets preloaded in that way.