WIP: New transition styles

I wanted to revisit our transitions and clean them up a bit. When we went to the box-shadow style iOS 7 transitions, we hit some performance issues. I tried a different approach that I think is better and lighter weight, and gets a similar effect.

Also, I've gone through and cleaned up references to `ios7` and normalized them to just `ios`. I did it in a simple way such that all the previous code should still work and it's backwards compatible.

Also, I added a new transition called `slide-full` that does a full slide rather than the offset slide on iOS, that I think is a better fit for Android.

Also, `<ion-nav-views>` has a black background and is positioned absolute with 100% width and height now by default which makes it possible to do a lightweight fade on transition. But I'm not yet sure of the full implications of this.

TODO:
- [x] Fix the border sizing issue that is causing a slight "reset" when finishing a transition
- [ ] Narrow down occasional back flicker (reflow?)