Minor core updates + Test cases for Command Manager plugin

As the title describes, I added some test cases for the new Command Manager plugin: I should have covered all the possible paths (but it may be worth double checking). 
I also moved the Game_event_router class into decorators.py: imho there is no need for a separate python module for this class since it's a decorator after all and I think it would be better to group them in a single module.