Add rss feed for public groups

Had a quick crack at adding some feeds; they're super easy, and work perfectly!

Except, we'll have to sit down and have a little think about how to password protect the non-public groups / discussions. (Suggestions welcome?)

I see benefit for having several things available as RSS feeds, including groups (a feed of new discussions), discussions (a feed of new comments), and motions (a feed of new votes)

In the meantime, hey, a feed of the `/groups` page would be pretty cool, no?

`/groups` on loomio:
![image](https://cloud.githubusercontent.com/assets/750477/2557748/f7e0842a-b718-11e3-9062-ffac450f639b.png)

`/groups.xml` in liferea (a linux desktop feedreader):
![image](https://cloud.githubusercontent.com/assets/750477/2557750/18288688-b719-11e3-8330-671b976bae66.png)

(note that the difference in order is because the public groups page gets ordered by # of members, while the feed is be ordered by time created)