Add structured data for holdings via http://schema.org/Offer

Add structured data for holdings via http://schema.org/Offer

Map library name to Offer/seller, shelving location to
Offer/availableAtOrFrom, call number to Offer/sku, barcode to
Offer/serialNumber, copy status to Offer/availability, and ISBN-13 to
gtin13. Use the additionalType of Product to give these offers an
obvious relationship.

Surface copy counts as AggregateOffer instances.

Cut over to RDFa Lite instead of microdata for schema.org, as RDFa Lite
is more easily extensible with other vocabularies, and is as accepted as
microdata by schema.org processors.

Signed-off-by: Dan Scott <dscott@laurentian.ca>