Multiple authors

Multiple author support is kind of a kludge, because sqlite3 stores lists as strings. It was either that or serialize them. So, I'm going to store multiple authors as just a plain, comma separated string (e.g. ignore list representation for now, as it would serve no benefit).

If functionality were implemented in the future that would benefit from list representation of authors, at that point it could be reimplemented so as to not break things from a user's point of view but make the change.