Add getRefersToTargetURI and getRefersToDate methods to Resource to make a cast to WarcResource unnecessary.
Addresses WWM-101. Old code commented out, clean up later.
provide default impl for getRefersToTargetURI and getRefersToDate in Resource, so that Resource subclasses compiles OK without change.
Merge pull request #36 from kngenie/WWM-101

Add accessor methods to Resource to make down-cast unnecessary