Restricts project poster to 1600px

I'm not super happy with this solution. If anyone has a better option, please contribute.

![screenshot 2014-06-24 15 17 26](https://cloud.githubusercontent.com/assets/1220232/3371543/0de8ea14-fba2-11e3-894e-e2afbd83e3c6.png)

Change log:
* Uses background-size: cover (like Medium) to scale the image
* Sets project poster as background-image for greater control
* Sets max-width of project poster to 1600px (dark background)