Generate a unique ID for iframes with no id attribute.

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))
Merge pull request #9 from ThibWeb/patch-1

Generate a unique ID for iframes with no id attribute.