add support for book covers (isbn only)

ensure title is always set, allow override

also use nice ouput on any non-xhtml renderer

major refactoring to have multiple ISBN resolvers

prefer full title for isbn

avoid slash on empty volume/issue

clean isbn before use

cache the query results

They are refreshed only when the code is updated.

fallback to editors if no authors

fetch DOI directly from doi.org

added ISBN support