Added copyright|databaseRight properties

This request addresses issues #9 and #12 adding some additional properties to the schema:

* copyrightYear & databaseRightYear -- year from which rights are claimed
* copyrightHolder & databaseRightHolder -- URIs that refer to rights holder
* databaseRightStatement -- equivalent to copyrightStatement

I've decided to retain copyrightNotice to allow users to express a copyright or database right notice with a Rights Statement. In practice there's enough variation in formatting (particularly with respect to rights holders) to make this tricky to generate automatically. I think getting buy-in from publishers will be easier if existing text can be directly added.