Unbind wasn't working for multiple key combos

The documentation shows that you should be able to call unbind with multiple shortcuts at the same time, just like in `assignKey`: `unbind('a, shift+b, ctrl+c')`. This was not working, since the `unbindKey` function simply didn't split the provided `key` parameter by comma (using the `getKeys()` function).

I've fixed this and also updated the unit tests. Basically, I've put a for loop around the current functionality to call it for each key combo that was provided.