Add updatedDate to item model

[schaal]

This is an updated version of #43, now that picoFeed has fallback values for pubDate (fguillot/picoFeed#293):

Implement your suggestions from owncloud-archive/news#982 (comment):

• update picoFeed to 0.1.28
• add updated_date to the database and bump version to 10.1.1
• use the new getPublishedDate and getUpdatedDate from picoFeed to set the corresponding item values
• adjust the unit tests

[BernhardPosselt]

Ok, lets see if this works :)

Thanks for the continuous hard work :)