Growl tweaks

* the content object is removed from the content array on timeout or close
 * the collection view takes care of removing the item view (cleans up the dom)
 * changed notification binding to be two-way, so when content is removed from collection-view, the GrowlNoticficationManager.notifications array reflects the same list.
 * cleanup regarding the comment I made to issue #36 and also implements the PR #53