Add tests for atom feeds.

Relates to #72.
Refactor atom feed generation.

Relates to #72.
Fix author uri for atom feed entries.

Relates to #72.
Date constructs in atom feed now contain ms.

Relates to #72.

As of http://tools.ietf.org/html/rfc4287#section-3.3 time should:
> Date values SHOULD be as accurate as possible. For example, it would
  be generally inappropriate for a publishing system to apply the same
  timestamp to several entries that were published during the course of
  a single day.
Fix feed author uri.

Relates to #72.
Fix atom feed self link.

Relates to #72.
Fix atom feed id.

Relates to #72.

__Note__: This fix is not backwards compatible as it changes the id for
each atom feed.
Add atom feed entry links to conversation and in-reply-to.

Fixes #72.

- link to html of entry has 'type=text/html' and 'rel=alternate'
- links to converation with 'rel=related'
- links to status it's a reply to with 'rel=prev'
Merge pull request #117 from mvitz/atom-feed-references

Add atom feed reference links.