ionSlideBox: automatically update when ionSlides are added/removed

**Proposal**
ionSlideBox automatically updates itself when slides are dynamically added/removed.

**Example of using this functionality**
```
<ion-slide-box>
    <ion-slide ng-if="showIntroSlide">...</ion-slide>
    <ion-slide ng-repeat="thing in things">...</ion-slide>
    <ion-slide>...</ion-slide>
</ion-slide-box>
```

**Bug Fixes**
Fixes #1578.
Possibly affects #1501 and #1473.
Should fix #1342.
Possibly affects #782.

**Integration**
It's now unnecessary to manually call `slideBox.update()` when slides are added or removed.  I'm not sure if there are other times when `update()` needs to be called, otherwise we could possibly deprecate it.  How should this change be messaged to developers?  Should we `console.log('Warning:...')` when `slideBox.update()` is called?

**Todo**
- [ ] run ionic tests
- [ ] write new tests
- [ ] handle this edge case where `setup()` clones some children -- what will happen if a slide is then added or removed?
```
      //special case if two slides
      if (browser.transitions && options.continuous && slides.length < 3) {
        element.appendChild(slides[0].cloneNode(true));
        element.appendChild(element.children[1].cloneNode(true));
        slides = element.children;
      }
```