Don't use 'use strict'; on the global scope

If this file will be concatenated with other files, it will enforce Strict Mode on all of the other code it's concatenated with.

Reference (under ```How do you use it?```):
http://www.nczonline.net/blog/2012/03/13/its-time-to-start-using-javascript-strict-mode/

By moving ```'use strict';``` inside the function, we can prevent that and still get the benefits.