improve the publication factory lookup 

improve the publication factory lookup by falling back to a more generic registration if the specific factory chooses not to handle the request after all. Also, fail clearly by raising an configuration error //always// if no handling factory can be (instead of returning None in that case, which is not handled properly in the callee).

Note that the tox-tests do not yet pass for me on Python 2.7. They do for Python 3.3. For some reason ``./bin/test`` on Python 2.7 do pass all tests. Don't know yet what is going on there.