Display navigation icon on iPad device-width

The navigation icon (3 horizontal lines) should appear on the right side of the header on iPad (see screenshot)

![screen shot 2014-01-17 at 2 09 06 pm](https://f.cloud.github.com/assets/3535767/1944938/07ac290e-7fc4-11e3-9dab-d7d6e7d1d3a7.png)