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).
Would it work to send back an empty array/collection? Generally I would avoid returning nulls or expecting nulls whenever possible, to avoid potential NPEs. Also consider using Guava's Optional<> class.