Allow for cross-origin prefetch in IE < 10

Since "real" CORS is not supported by jQuery in IE < 10
(see http://bugs.jquery.com/ticket/8283#comment:1) the only options for
supporting cross-origin fetches in IE are

1) using one of quite a few plugins which extend jQuerys transport layer in a
   way that makes use of IE's XDomainRequest object (see above jQuery ticket)

2) using JSONP (with capable backend)

By adding the dataType option to prefetch data sources and making use of the
more low-level $.ajax() call, we can offer JSONP support for prefetches at no
additional (frontend) cost – it goes without saying that the prefetch backend actually has to support JSONP wrapping in this case.