Adding a formset to a form a la django dynamic formsets

Not sure where to send this for a pull request. webtest seems django-agnostic, so here seemed best.

We (I work with Julien Aubert) had a test where there was javascript in the page to add a formset form with some data. In the browser this was done with javascript, using the django dynamic formset mechanism. This wasn't reproducible in webtest due to it not handling the javascript, so this code does the same as the javascript to enable the test.

No problem if you want to decline the request, just offering it in in case it might be useful for those in the same situation.