code: function get(url, on_success) {
          var on_error = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : null;
          var headers = arguments.length > 3 && arguments[3] !== undefined ? arguments[3] : { 'X-Requested-With': 'XMLHttpRequest' };
  
          this.create('GET', url, {}, on_success, on_error, headers, true);
      }
docs: Sends a simple GET request. By default adds header X-Requested-With: XMLHttpRequest
  which allows back-end applications to detect if request is ajax.
  However for making a cross domain requests this header might not be acceptable
  and in this case pass an empty object {} as a last argument to send no headers
  
  @param {string} url - URI or full URL for cross domain requests
  @param {callback(responseText)} on_success - callback for status code 200
  @param {callback(responseText, responseStatusCode)} on_error = null - custom handler for non 200 status codes
  @param headers = {'X-Requested-With': 'XMLHttpRequest'} key: value map of headers