fix add_zoom_to_images to pass the image class to the wrapper div

Bráulio,

   This is a fix for the problem with the thumbs because of the zoomable-image javascript. The way I fixed is rather simple, but I wasn't able to test it because now that I put the noosfero/stable, I cannot test anymore. And I cannot run it anyway because I erased the database.yml, I kow it was stupid, but since it wasn't being tracked I thought it should not be there. Now I can't even run tests.

    Ah: a good thing: this change I did don't change any tests, so it can be merged to noosfero with the rest of the commits of the automatic_abstract.

        daniel