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>
```
Can we make this a bit more readable?

```ruby
t('cookbook.activity', 
  maintainer: link_to(cookbook_version.cookbook.maintainer, cookbook_version.owner),
  version: cookbook_version.version,
  cookbook: link_to(cookbook_version.name, cookbook_version.cookbook).html_safe,
)
```

Also, why no `.html_safe` on the maintainer link?