Refactoring and changes to AssetManager class

AssetManager class was calling the load callback before all assets had
been loaded.

Changed addImage to addAsset and imageLoaded to assetLoaded. These now
account for audio and json data (not just images).

Changed upToDate to function isLoaded.

objects.json and map.json are now loaded as assets. Before they were
loading out of sync with the other assets. This could cause a problem
with the objects.json and map.json taking longer to load than the
assets, and therefore the load callback being called too early.