rdf: support tied genid output between graphs
api: document the trie data structure
api: document the path APIs
metadata.dox: update the documentation for the new APIs
document.dox: update the documentation for the new APIs
dictionary.dox: update the documentation for the new APIs
espeak/en: add more phonemes to the phonemeset
epub: pass ncx table of content metadata through
document: add a constructor to read metadata in the document body
metadata: add an rdf::query::literal helper function
document: use Document Reference metadata for the table of content if present
pdf: generate Document Reference metadata for the table of contents
pdf: update the copyright statement
pdf: ensure that the toc list is properly terminated
rtf: generate Document Reference metadata for the table of contents
rdf: rename rdf::graph::foreach to rql::rdf_list_items
ncx: move the title toc-entry event to RDF metadata
plaintext: generate Document Reference metadata instead of toc-entry events
opf: rework to process spine nodes during read()

The parser stored the elements in the spine as a list of ids into
the fileinfo (manifest) map. This change reworks the logic to
process spine nodes during the read() calls, removing the need to
store the spine data internally.

This is the first step to get the OPF parser to generate Document
Reference metadata for the spine data.
Use kramdown to perform the markdown => html conversion.