Fix navbar arrows for Safari

This fixes a bug with the "arrows" in the navbar under Safari (thanks @jebeck for pointing it out).

Man.. these arrows are the worse :p Tried to keep the CSS as clean as possible, but it's still a bit of a hack and magic numbers. @skrugman Just pointing it out for the future, some things are easier to build & maintain than others in HTML & CSS :)

BEFORE:

![screen shot 2014-09-16 at 9 35 14 am](https://cloud.githubusercontent.com/assets/1306536/4288713/43c1e2be-3dae-11e4-8090-24b823c39d45.png)

AFTER:

![screen shot 2014-09-16 at 10 28 03 am](https://cloud.githubusercontent.com/assets/1306536/4288719/4ccc246e-3dae-11e4-8b77-db07f496a38b.png)