Support view modes for (image) ads

For image adds i wanted to use different view modes depending on the ad group so I could change the size of the image depending on the group. I added a new text field "field_adgroup_viewmode" to admin/structure/taxonomy/ad_groups/fields so I could enter the view mode.

Attached patch contains the code changes, the install isn't changed yet, if you like the idea I'll update the patch so it includes the new field.