items.php edited timestamp logic

This concerns the check for whether an incoming feed item should overwrite the item in the database.  At the moment it's too aggressive.  It'll compare the existing timestamp "2013-03-20 09:04:02" with the incoming timestamp "2013-03-20 09:04:02Z", conclude that they're different, and overwrite, because it's just doing a simple string comparison.  That seems just plain incorrect.

It's not clear what it should do if the incoming timestamp is older than the existing one.  It seems safest to not overwrite in that case.

items.php could do with some refactoring :-(