Add background image to issue icon

This adds (and populates) a field for the background image of issues' icon. Also, a simple inline-style to display it.

Images are already inside the shared media directory and s3.

E.g. 
![screen shot 2014-12-18 at 6 23 58 pm](https://cloud.githubusercontent.com/assets/326918/5498328/3025339a-86e3-11e4-8f00-25146f213a96.png)