[WIP] Zend\Feed fixes (#2969)

Fixes for issue [2969](https://github.com/zendframework/zf2/issues/2969):
* Add missing methods in Zend/Feed/Reader/Extension/Atom/Entry.php (getCopyright(), getContributors())
* Add missing methods in Zend/Feed/Reader/Extension/Atom/Feed.php (getIcon(), getContributors())
* Rename Zend/Feed/Reader/Collection/Author.php to Zend/Feed/Reader/Collection/Person.php (to follow Atom 1.0 Specification where Author and Contributor are "Person")
* Replace all old references to Collection\Author by Collection\Person
* Add units test for Zend/Feed/Reader/Extension/Atom/Entry.php and Zend/Feed/Reader/Extension/Atom/Feed.php (testing new methods)