EPUB parse dc:identifier as URI, infer from opf:scheme or the refine pattern

[danielweck]

See:
https://github.com/readium/architecture/blob/master/streamer/parser/metadata.md#identifier

r2-shared-js/src/parser/epub.ts

Lines 1144 to 1156 in a01efc9

	const addIdentifier = (publication: Publication, _rootfile: Rootfile, opf: OPF) => {
	if (opf.Metadata && opf.Metadata.Identifier) {
	if (opf.UniqueIdentifier && opf.Metadata.Identifier.length > 1) {
	opf.Metadata.Identifier.forEach((iden) => {
	if (iden.ID === opf.UniqueIdentifier) {
	publication.Metadata.Identifier = iden.Data;
	}
	});
	} else if (opf.Metadata.Identifier.length > 0) {
	publication.Metadata.Identifier = opf.Metadata.Identifier[0].Data;
	}
	}
	};

r2-shared-js/src/parser/epub/opf-identifier.ts

Lines 28 to 29 in a01efc9

	@XmlXPathSelector("@scheme")
	public Scheme!: string;

r2-shared-js/src/parser/epub/opf-metadata.ts

Lines 36 to 38 in a01efc9

	@XmlXPathSelector("dc:identifier")
	@XmlItemType(Identifier)
	public Identifier!: Identifier[];

r2-shared-js/src/parser/epub/opf-metafield.ts

Lines 31 to 38 in a01efc9

	@XmlXPathSelector("@refines")
	public Refine!: string;
	@XmlXPathSelector("@scheme")
	public Scheme!: string;
	@XmlXPathSelector("@property")
	public Property!: string;

Related issue:
https://github.com/readium/r2-streamer-swift/issues/113