Fix set_group when no group is specified

Some pages that list contacts (e.g. when composing a message) issue a
set_group(0) on the addressbook object.

That means not restricting the search to a group at all, but the plugin
was trying to search for users belonging to a group with id = 0.
Merge pull request #16 from adobo/fix_listing

Fix set_group when no group is specified