TabBar badges support

Added support for TabBar icon badges using the attribute badge in the tab directive.

```html
<tab icon-on="icon ion-ios7-person" icon-off="icon
ion-ios7-person-outline" href="#/tab/myprofile" badge="3">
```

I’ve used the colors of the TabBar itself but inverted for the badge. I don’t know if you’ll want to merge this straight on but I think it can help as a start point for this feature. I’ve tried to do it as integrated as possible.