Observer link

Show a link to the observer for ( non series ) observer articles in a similar way to how we signpost series. This change also refactors the associated css

![observer-branding](https://cloud.githubusercontent.com/assets/986155/2696594/73592b8e-c3e4-11e3-9ac6-a8b92ef34f50.png)
![series-branding](https://cloud.githubusercontent.com/assets/986155/2696598/7ee53006-c3e4-11e3-87e0-c36c1558a8da.png)