feat(loaders): add jsonp support

I needed jsonp support so I did this small patch. Very easy to use. Just pass `method: 'JSONP'` as an option on `$translateUrlLoader`.

```javascript
$translateUrlLoader({
  key: 'de_DE',
  url: 'foo/bar.json?callback=JSON_CALLBACK',
  method: 'JSONP'
 });
```
```javascript
$translateProvider.useLoader('$translatePartialLoader', {
  urlTemplate: 'https://your-site/translation/{part}?locale={lang}&callback=JSON_CALLBACK',
  method: 'JSONP'
});
```
Works on loader-partial and loader-url. 

Cheers.