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).
I moved these commands from CoreCommands in a seperate class, didn't know how to name the class. There may be a better way to categorize the commands in classes ClientCommands and ServerCommands.