Add support for Vertical Tabs

Views attached cannot easily be put into a fieldgroup (as noted here: #363969: Allow Attached be part of Field Group). I suggest supporting Vertical Tabs as a workaround. This could easily be made to be configurable, but my thinking is if Vertical Tabs is already in use on a site, it might actually make more sense to just support it out of the box.

This patch simply checks for Vertical Tabs and if it's found, makes the Views Attach form item a fieldset rather than just an item.

Note, it was created following the guidelines for doing so here: http://drupal.org/node/1054616