WIP: Popup Support

This is a WIP PR for adding popup support to Ionic. When I say Popup, I mean a popup dialog with customizable buttons and a message:

![popup](https://developer.apple.com/library/ios/documentation/userexperience/conceptual/transitionguide/Art/alert_7_2x.png)

- [x] Customizable buttons
- [ ] Custom size?
- [ ] Enumerate use cases