OPA with ottobib
Inspired by LibraryThing I've added an OttoBib redirector to the Isbn provider in OPA. It seemed a good time to refactor some, so I did that, too. The FORMATS/EXTRA_FORMATS dichotomy is artificial and just a remnant, now, but I still don't have time to rewrite the whole thing. In the meantime, it's slightly more sane inside the Provider subclasses.
Updated opa.py is attached.