First set of exercises for events & callbacks

Includes basic outline for Events & Callbacks page and a set of exercises on Passing Functions to Functions.

Mostly focused on practice with the syntax of passing named & anonymous functions as arguments, so it is more of a mental exercise in building the notion of functions as objects than an exercise in learning new terms or methods.
Is there a reason you're using inprecise/uncertain language here? "maybe" "some modal"?