BibAuthorID: hot fix for optional multiprocessing

• Hot fix for mandatory import of multiprocessing library introduced by 4eda63716e2523cab94bb5f74037ecf731c9b2bf. The multiprocessing library is not available on Python-2.4 by default and it is not a required dependency, so it may not be present. BibAuthorID should check for its presence and work in a 'degraded' mode without it, via a technique like MP_ENABLED used elsewhere in the BibAuthorID module already. This hot fix simply passes when multiprocessing is not installed, so that Invenio instance would continue to work and would not traceback.