VuFind 425 - Add schema.org structured data for bib records and holdings

Per http://vufind.org/jira/browse/VUFIND-425, add mappings to schema.org structured data via RDFa Lite for basic bibliographic record attributes and holdings information. This builds on Eoghan Ó Carragáin's initial work and is informed by the most recent W3C Schema Bib Extend community group discussions on holdings.