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`