Fixes #3: emit entityAdded event when entity added
Fixes #2: entities now have name property
Fixes #4: start update loop immediately.

Also adds two events: "preload" and "preloadComplete". Entities are not
subscribed to game's "update" event until after "preloadComplete" has
fired.

This should make adding preloaders a lot easier since they can rely on
the game's update/draw cycle.