feat(ngAria): Announce ngMessages with aria-live

To be accessible, error messages shown with `ngMessages` should be read aloud in a screen reader. This can be easily accomplished by adding `aria-live="assertive"` to the `ngMessages` directive via `ngAria`. 

Proof of concept in this Plunkr: http://plnkr.co/edit/6iFNWiSiRWz9y871v7oq