Start adding unit tests

Using QUnit 1.12.0 from http://qunitjs.com/
Bug fix: an empty array doesn't == false.

The require function was mistakenly assuming that an empty array would
be considered equal to false when used in an if-test. The fix is to
check whether the length of the array is greater than zero instead.

This is the first bug caught by the unit tests. Hooray!
Fix mistaken use of useKeyboard in mouse.js

Should have been returning useMouse, but was trying to return
useKeyboard. Copy 'n' paste error.
Add mising 'ludum.' before the hasNameClashes function in tests-base.js
Change the test page title to "LudumEngine Unit Tests"

That seemed a bit better than "QUnit example" :-)
Add a 'Testing' section to the readme.