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.
Though it may be easier for people to get "doThisXTimes" instead of iteration over an array. Maybe that could be a pre-cursor here.