UI improvements and JS cleanup

Hi pp,

Thanks so much for the work on the module, it’s saved me tons of work!

I’ve modified the JS file a little to clean it up a little and improve performance. I’ve also added these features:

• Added extra classes to the "Add another" button to aid themers.
• Change the button text depending on the number of uploads ("Add media" vs. "Add another")

Let me know your thoughts, patch is forthcoming.