Implement atom feeds

Implement Atom feed of activities.

![gitbucket](https://f.cloud.github.com/assets/1453749/2499417/14498736-b356-11e3-8766-cbc2e8cac7c9.png)

Get feed link icon 'feed.png' from takezoe's nicocale project.
[ニコニコカレンダー Project Top Page - SourceForge.JP](http://en.sourceforge.jp/projects/nikocale/ "ニコニコカレンダー Project Top Page - SourceForge.JP")