Fix Timestamp publish issue #3721

* Added function field publish_date in article model.
* Replaced published_on with publish_date in tree view.
Merge pull request #34 from tarunbhardwaj/task-3721

Fix Timestamp publish issue #3721
Added classmethod on get_publish_date
Merge pull request #35 from tarunbhardwaj/task-3721

Added classmethod on get_publish_date