Changes the use of the 'use strict' pragma

This change wraps the module and directive declaration with an anonymous function that should limit the scope of the 'use strict' pragma to the angularjs-dropdown-multiselect module.

Not limiting the scope of the 'use strict' pragma in this way can be problematic for projects with build processes that combine several javascript files into one (for deployment purposes) if some of the files contain code that is not expecting to be running with the 'use strict' pragma.