atom-feed content changed to html, use better post-title extractor

Valid atom feed. Also these features are partially included:
- http://activitystrea.ms/specs/atom/1.0/#activity.verb
- http://portablecontacts.net/draft-spec.html#anchor16
Merge pull request #4834 from Faldrian/atomfeedtohtml

atom-feed content changed to html, use better post-title extractor