Add README for events_and_callbacks

Readme with links to glossary entries on events and callbacks.

Lists link to (currently empty) exercise on passing functions to
functions.
Add exercises for passing functions to functions

Mosty focused on practice with the syntax of passing named & anonymous
functions as arguments.
Add description of events to events_and_callbacks/README

Short description of events with an example.
Fix links to glossary entries in events_and_callbacks/README
Copyedits to description in README per @zspencer's feedback
Hard wrap lines on *.md at 80 chars
Use more meaningful example for passing functions as arguments exercise
Use more nuanced exercises for passing function with arity 1

As per @zspencer's feedback, this keeps the focus on solving problems
that deal with the topic at hand (passing and executing functions that
take their own arguments).
Start multiple callback exercise with a more basic iterator
Fix bug: unnecessary '#' in argument to `getElementById`
Include full HTML for example of event listener
Merge pull request #3 from codeunion/passing-functions-to-functions

First set of exercises for events & callbacks