fix(navBar): remove flickering/bumping for back button

Tested on Android 4.0, 4.1, 4.2, 4.4, iOS7, iOS7 Simulator

A few changes:

1. Use ng-show/ng-hide for back button, so there's no possibility of it flickering when it is created each time
2. Hide back button by default
3. Add styles for ng-hide/ng-show in nav-title-slide
4. Make it so only the title translates on transition
5. Make titles only be invisible on start **if there's an animation on the navbar** (using css & new `title-animate-no-flicker` class).  This stops the titles being blank then appearing for a sec on old android when there's no animation.
6. When you press an icon-button (the back-button), it goes to 0.3 opacity.  Before, you would press it, see 0.3 opacity, let go, then see it go from 1->0 opacity. It looked weird (0.3->1->0). Now when you press the back button it adds the active class to it for the duration of its animation and the back button just goes from 0.3->0.
7. Fixed #624. In chrome, there's a bug where items transitioning on opacity 'bounce/bump' a bit while changing. http://stackoverflow.com/questions/12980153/  It was fixed by putting backface-visibility: none on button-icon & button-clear buttons.  I tested this and it *does* fix the problem, and presents no other problems I could find (unless people *do* want backface visibility!). Lemme know if this is bad, you can see it in the scss, and we can remove it and just ignore the bug if you wish (it's in desktop Chrome only, that I could find)