handling labels on the Groups DOM node to update the model
added handling to the post-it DOM labels in a similar manner as the Groups DOM nodes.  Pressing enter will update the model in the same way clicking away from the label will
REFACTOR: seperated concerns by pulling out offset calculation and group DOM element heigh into helper functions to be called by the group_drop_handler.  This will also allow for other events to trigger an update to the group's DOM node--not just a post-it drop
add a removePost() and a corresponding helper function to the Post_Group model