Fix #27. Sort activity stream.
Add a check if a person has friends.

In cases of not having friends an error in template occurred
caused by an unset array after homeAction of Xodx_UserController.
Fix #28. Enabled preview of images.

Previews are shown in activity streams and resource views. Still the need of enabling tagging of persons on images as described in Issue #21
Merge pull request #37 from splattater/fix/friendlistActivitystreamImage

Fix friendlist activitystream image