Added the addListener method.
Added JSDoc to current Events methods.
Structured events tests and added a new one.

So now I check to make sure you can't add multiple listeners for the
same function.
Added the removeListener method and tested.
Added sinon and emitting to Events.

Also tested emitEvent a lot.
Used the right method for a removal test.

Now it uses notProperty to make sure test-event is not a property of
events.