No asset handling

[BillyWM]

The asset handling system will eventually handle pre-loading all images, sounds, etc, and providing a convenient reference to them.

A single Assets object will hold references to all assets, for example: Assets.images.playerImg

This loader should provide preloading information. At the very least, it should periodically report the number of files loaded / total so developers can build a preloading screen with this information. Getting the bytes loaded / bytes total is problematic (cross-browser issues) but will be considered for future inclusion.