add ajaxContentType config parameter

Add ajaxContentType parameter to allow configuring of the content type of the
ajax request sent to the server for getting the recurrence occurences.

This change is needed for a working integration in plone.formwidget.recurrence.

This is why:

When trying to get the ajax data (which is sent via POST and of the ```application/json``` ContentType) from the request in json_recurrence via ```self.request.form``` it isn't there. Of course, because it should be in the body. But it isn't there either: ```self.request.get('BODY')``` returns an empty string. Maybe I misunderstood the ZPublisher.HTTPRequest API (likely) or ZPublisher is just missing correct handling of json POST requests (also likely).

Anyway, by making the contentType parameter of the ajax request configurable, I'm able to set it back to ```'application/x-www-form-urlencoded; charset=UTF-8'``` (the default, as it was before). Then everything works again...

Correct me, if I'm wrong...