Made it a UMD module

[mjsarfatti]

This polyfill seems great, I actually did 3 thing:

1. made it into a UMD module, so it can be imported in bigger projects
2. by consequence as a module it exports an init() function, so that it can be called on dom ready (instead of being immediately executed)

$(document).ready(function() {
    objectFitPolyfill.init();
});

1. incorporated Fixed bug when class was not copied #2