Fixed header background on large screens

Hey,
On large screen there is an issue with header background. I'm attaching screenshots before and after my fix. Hope you like it.

Best regards,
Oskar
![before](https://cloud.githubusercontent.com/assets/5034850/4034972/6af91166-2c8a-11e4-974c-57d1c8160530.png)
![Uploading after.png . . .]()


If we're not bothered about fixing this bug for old IEs using large screens, which I'd vote we're not, then the current CSS `background-size: cover;` will be fine, changing to use technique#2 will only add support for IE8 over the CSS3 option.