Tabs: Use fade animation on tab contents

Adds fade="true" to tabset.

```html
<tabset fade="true">
	<tab heading="Fading Tab 1">
		<b>First</b> Faded Content!
	</tab>
	<tab heading="Fading Tab 2">
		<i>Second</i> Faded Content!
	</tab>
</tabset>
```

Requires ngAnimate and additional CSS:
```css
.tab-content > .tab-pane.fade.in-add {
  display: block;
  opacity: 0;
}
.tab-content > .tab-pane.fade.in-add.in-add-active {
  opacity: 1;
}
```