Use css background for jumbotron and move title header into it

I've changed the layout a bit:
- there is a `page.image.feature_class` with default value `jumbotron-bright` that uses css gradient
- the title header is now moved up and displayed on top of gradient background

![screen shot 2014-11-01 at 09 03 25](https://cloud.githubusercontent.com/assets/357191/4871102/9ff62e7e-619d-11e4-874d-cc5e86880525.png)