Implemented Sprite system for stuff that can interact with the player.
This will be used for potions, money, enemies and is already used for
springs.

Also implemented FlexiblePlatform, wich is a platform that uses
PlatformListeners in order to allow easier combined platforms.
There are currently no implemented Listeners, but if they were
implemented you could combine a breaking platform and a moving platform
by applying the right interfaces.

Maybe I will also implement something to load JavaScript platform
listeners and Biomes.