Added ionModalView

Makes adding a modal a little more Ionic Framework-y - Closes https://github.com/driftyco/ionic/issues/1667

Anything thoughts, comments, or critiques are welcome - this is my first JS contribution to the world :)

**OLD WAY**
```html
<div class="modal">
  <ion-content>
    Josh is a pretty cool guy
  </ion-content>
</div>
```

**NEW WAY**
```html
<ion-modal-view>
  <ion-content>
    Josh is a pretty cool guy
  </ion-content>
</ion-modal-view>
```

**OR OTHER NEW WAY**
```html
<div ion-modal-view>
  <ion-content>
    Josh is a pretty cool guy
  </ion-content>
</div>
```