added jsonp for custom url

I noticed this pull request adding the ability to have a custom url: https://github.com/emberjs/data/commit/35f66f12781af18187569ae1f70651754136f8ac to your RESTAdapter. In using this, I noticed that the dataType of json was still being used, but when a custom URL is used, it should be using jsonp for CORS support.

I added dataType of jsonp when a custom url is used.