epubextractor: Add property ReleaseYear
Summary: Make epub-file properties more consistent with other media types which all provide ReleaseYear.
Test Plan: make test
Reviewers: astippich
Reviewed By: astippich
Subscribers: ngraham, bruns, kde-frameworks-devel, astippich, Baloo, mgallien
Tags: Frameworks, Baloo
Differential Revision: https://phabricator.kde.org/D11869