Generate a unique ID for iframes with no id attribute.

Problem: `var frame = document.getElementById(base.$el.attr("id")); ` will not work for frames with no id attribute.

Solution:
Add a unique ID generation process for frames with no ID. The ID generation is taken from the .uniqueId() function in jQuery UI core (see the [source of jQuery UI core](https://github.com/jquery/jquery-ui/blob/master/ui/jquery.ui.core.js))