API "return" inconsistencies

The way different entities in the API handle the "return" is really inconsistent. $params['return'] was ignored for Activity/get with regard to targets and assignees--it had to be return.target_contact_id. When it's a comma-delimited list, spaces after the commas cause problems. If it's an array, the first item is ignored (it has the key of 0, so using array_flip, the value was set to 0). I've attached a patch to fix these, and I also enabled sort, offset, and limit for group/get.