Covers in results view (#172)

Works with iOPAC and Zones (tested for Stadtbücherei Schleswig, Bücherhallen Hamburg). The other OPACs don't show cover images on results pages as far as I could see, so I didn't add it for others.
It is loading all the cover images on one page before showing the results, but I didn't notice a huge difference in loading time.
I don't know if it is okay to put the cover instead of the media type images when there's a cover available (then there's no indicator about the media type anymore) and maybe we could make the ImageView a little larger for the covers.