code: public PayloadBuilder customField(final String key, final Object value) {
          root.put(key, value);
          return this;
      }
docs: Sets any application-specific custom fields.  The values
  are presented to the application and the iPhone doesn't
  display them automatically.
  
  This can be used to pass specific values (urls, ids, etc) to
  the application in addition to the notification message
  itself.
  
  @param key   the custom field name
  @param value the custom field value
  @return  this