Use a px text-indent on .pika-prev/next to prevent Firefox bug

This bug https://bugzilla.mozilla.org/show_bug.cgi?id=908706
make the first letter of this buttons visible.

Before

![screen shot 2014-01-24 at 17 13 16](https://f.cloud.github.com/assets/157534/1996429/0ff5915e-8514-11e3-8995-56717a755b38.png)

After

![screen shot 2014-01-24 at 17 25 20](https://f.cloud.github.com/assets/157534/1996446/3b8a7ec4-8514-11e3-9142-601405e8080a.png)

Approved on Gecko & Webkit browsers.