Make followed cookbook activity feed conform to atom spec

:fork_and_knife: Move maintainer information into author tag, remove non standard tags. This is what an entry now looks like.

```xml
<entry>
  <id>tag:localhost,2005:CookbookVersion/5386</id>
  <published>2014-06-24T15:28:26Z</published>
  <updated>2014-06-24T15:28:26Z</updated>
  <link rel="alternate" type="text/html" href="http://localhost:3000/cookbooks/nginx/versions/0.1.0"/>
  <title>supermarket released v0.1.0 of the nginx cookbook</title>
  <author>
    <name>supermarket</name>
    <uri>http://localhost:3000/users/supermarket</uri>
  </author>
</entry>
```
Yeah should be added to maintainer as well. :+1: