Change exercise to require students to implement IIFE and invoke their own module

[jaywon]

Lecture slides show module pattern as an IIFE that's output is stored in a variable but the exercise just has them write a function that the test invokes and then calls methods upon.

We should be consistent and have the students invoke their own module and tests should just run against the global module variable.

Also, may want to add a note to pay attention to the tests to know which methods the module needs to reveal.