Create recurring allocations with plone.formwidget.recurrence.

This PR adds the possibility to create recurring allocations using an RRULE based recurrence widget.

:construction: Since the recurring allocations are all generated when the form is submitted we must disable the functionality for never-ending recurrences. I'm waiting for the following PR's to be merged:
collective/jquery.recurrenceinput.js#17
collective/jquery.recurrenceinput.js#18
As soon as the above PRs are merged i will issue a follow-up PR to `plone.formwidget.recurrence` to include the updated javascript file and the widget parameters.

Therefore at the moment the new functionality used in this PR is only available in our `plone.formwidget.recurrence` fork at `github.com/4teamwork/plone.formwidget.recurrence.git`.

@href Please have a look! (I have not updated the buildout configuration with the sources mentioned above yet as i'm not sure about how you want to proceed.)