Improve ContributionPanel style in OS X

* Improve contrast for ContributionPanel selected style
* Use roundRect buttons in ContributionPanel

![captura de pantalla 2014-11-27 a las 22 23 16](https://cloud.githubusercontent.com/assets/138426/5222808/42b5a89c-7684-11e4-8c30-91174ecc8973.png)