code: public InstagramSubscription callback(String callback) {
  		Preconditions.checkValidUrl(callback, "Invalid Callback Url");
  
  		this.params.put(Constants.CALLBACK_URL, callback);
  
  		return this;
  	}
docs: Configures the callback url
  
  @param callback The callback for your application
  @return the {@link InstagramSubscription} instance for method chaining