Added support for titles and stacking direction.

With defaults, no changes take place. Use option stack:'above' to
prepend new notifications to the stack, while the default 'below'
appends them as usual. The message object now can contain a new key,
"title" that contains the title TEXT (i.e. no html). The title tag can
be controlled by the additional option "titleTag" which defaults to
"<h4>".