Update drupalgap.js

Added support for images stored with Amazon S3 module.

You can define in settings.js, file_public_pat variable with the path to your cloudfront or bucket url.
Ex:
/* Files */
  'file_public_path':'sites/default/files',
  's3_public_path':'http://museion.s3.amazonaws.com',

https://drupal.org/project/amazons3
"The AmazonS3 module allows the local file system to be replaced with S3. Uploads are saved into the drupal file table using D7's new file/stream wrapper system."