Use loader with jsonp

I love angular-translate and this loader. So, thank you very much for these projects.

That being said, I needed jsonp support so I did this small patch. Very easy to use. Just need to pass callback on the urlTemplate.

` urlTemplate: 'https://your-site/{part}?locale={lang}&callback=JSON_CALLBACK'`

See if you think is worth it to put on the official repo.

Cheers.