improve post hook implementation in EntityTag.php

attached patch improves a few things in how Core/BAO/EntityTag.php implements the post hook

• it ensures post hook is called for all instances of add/delete (this may have already been patched in 3.4/4.0)
• it alters the array of contact ids passed to the hook so that only those ids acted upon are passed – not the full list of ids from the search results
• it adds additional conditions to single contact tag remove so that removal only is fired if tag previously existed