Fix jump in navigation toggle animation

By moving the `padding` from `.navigation`, the element being animated, to an interior element, we can avoid the jump at the end of the navigation toggle.

Screencast comparing navs: http://f.cl.ly/items/463t401X0n3K1X2D1O1o/Screeny%20Video%20Nov%2029,%202013,%202.13.20%20AM.iphone.mp4