+1 for @brandyshea 's answer.
Use the bar-clear class for the nav bar. But you also have to add some styling to the ion-content directive otherwise you can get an empty white bar at the top.
<ion-view>
<ion-nav-bar class="bar-clear"
...
</ion-nav-bar>
<ion-content class="no-header" style="top: 0;">
...
</ion-content>
</ion-view>
Here I have added some inline css but its better to add it to your scss file