Add related ISBNs to meta data

[alerque]

Figure out a way to splice in related ISBNs (such as of original language versions, alternate printings, etc.) into the meta data. This can be used in the play books related ISBN field, for building website listings, and so forth.

[alerque]

EPUB meta data uses a source key for information about original language sources, probably just replicate whatever data we have in the same tree format as the TR under a source: key...