Reworked the command registration system

Please read the [javadoc](https://github.com/Limeth/Terasology/blob/16965803d4edd128ce5baaa3c35e814de26752f3/engine/src/main/java/org/terasology/logic/console/Command.java).
Suggest use of Preconditions to check parameters and state. Should not silently work when sent null, and probably shouldn't fail open (return true when sent null).