fixes display problem with box-sizing: border-box

When box-sizing: border-box was used (like shown here: http://www.paulirish.com/2012/box-sizing-border-box-ftw/), parts of the player (e.g. play-button) looked weird/not as they should.

Put this in an own CSS-rule so it can easily be reverted when the player CSS is switched to use border-box everywhere.

works around #109

Before:
![box-sizing-before](https://f.cloud.github.com/assets/164417/968706/27467bae-05ab-11e3-9082-6ce8f170376d.png)

After:
![box-sizing-after](https://f.cloud.github.com/assets/164417/968705/272eb1f4-05ab-11e3-9da1-5ea352493b52.png)