Add removeHotkey to keyboard module, closes #110

[thomsbg]

Takes a string, number, or hotkey object, returns the handler function.

[jhchen]

Can we toUpperCase() a string to support case insensitive removals ex. removeHotkey('bold'); Many users might not and need not know the hotkeys object has its keys in all caps.

[jhchen]

We had discussed that the handler for a hotkey be returned by the removeHotkey function but there may be more than one. Should it just remove and return the first or remove and return all of them? The latter of removing all of them makes more intuitive sense but returning a different type (an array) is not.

[thomsbg]

@jhchen I've added another commit which addresses your concerns (and actually makes it work!)

I decided to rename the method to removeKeys (plural) to indicate than the return value is an array.

I also added the ability to specify which handler you wanted removed, along the lines of jQuery's off(event, handler).

[jhchen]

Thanks for the changes!