Added a quickfix to center iframes (youtube video) + responsive stuff into css

Before
![capture d ecran 2014-02-10 11 31 53](https://f.cloud.github.com/assets/1264761/2127685/cabdca82-9271-11e3-8a2f-ef4823ad46a8.png)
After
![capture d ecran 2014-02-10 11 31 23](https://f.cloud.github.com/assets/1264761/2127686/cc219854-9271-11e3-9ec4-5bc7f4e5dc1c.png)