Feature: Added mockjax function `unfiredHandlers` and associated test

The `unfiredHandlers` function returns an array of all the `mockHandlers`
that are unfired. This makes it easy to find cases of unnecessary mocking
in your tests.
Feature: Updated README to show `unfiredHandlers`
Merge pull request #160 from JarrodCTaylor/master

Feature: Added mockjax function `unfiredHandlers` and associated test