Add a new format for the RSS feed.

#4279 

Output in title view:
![selection_062](https://f.cloud.github.com/assets/4497748/1629952/59ed2094-5740-11e3-8e97-9dab95eea6d0.png)

Output in teaser view:
![selection_061](https://f.cloud.github.com/assets/4497748/1629954/662617bc-5740-11e3-95d5-498ac484b148.png)