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).
To be really clear, the entire CommandRegistry class is unnecessary. You can use a standard HashMap<Name, Multiset<CommandInfo>>, as Name gives you case-insensitivity.