Change properties for abstract and bibcode

As @lanthaler [pointed out to me](https://twitter.com/MarkusLanthaler/status/517676273888264194), the previous ones weren't properties. Abstract now comes from dcterms. I'm still not sure about bibcode. The best I could find was [http://info-uri.info/registry/OAIHandler?verb=GetRecord&metadataPrefix=oai_dc&identifier=info:bibcode/].