Return a close promise from open()

This request supersedes my request adding a return to ngDialog (#40), but I wanted to separate them since I know that this addition might be more controversial. :)

I've added a `closePromise` to the return object from `open()` which will resolve when the dialog is closed. Useful for being notified regardless of how the dialog is closed without needing to set up an event listener and filtering out the dialog in question.