Add function for registering event hooks.

[RobertWHurst]

I will be adding a function that allows a user to register a function that will run each time the "executeActiveKeyBindings" is executed. This will allow people to watch the internal state of KeyboardJS and inject additional functionality.

I'm open to suggestions on how what kind of arguments one would expect the user's hook callback to be passed.

[RobertWHurst]

This is something that I want to get right so I'm going to hold off until I have a solid understanding of how and where to add hooks. I don't want to enable access to the core if programming ninjas are going to do things in there that they shouldn't.