Add Offer schema.org markup

This adds schema.org Offers to the output. You can see this in effect here:

http://plugins.leewillis.co.uk/downloads/woocommerce-pro-shipping/

http://www.google.com/webmasters/tools/richsnippets?q=http%3A%2F%2Fplugins.leewillis.co.uk%2Fdownloads%2Fwoocommerce-pro-shipping%2F