713 Fix wrapping in navigation

Prevents the navigation from breaking the layout for smaller window sizes (https://github.com/gregbell/active_admin/issues/713). The navigation entries now wrap properly without overlapping each other (and the h1 and utility-nav):

![navigation](https://f.cloud.github.com/assets/563038/333059/fdf12bc0-9c44-11e2-90f5-2f7373b2960c.png)