BibClassify: remove ability to run as standalone

* Removes ability to run BibClassify in standalone mode.

* Fixes an issue where lib/python/invenio/ was added to sys.path to be
  able to run e.g. BibClassify as standalone. This caused modules to
  be importable outside of their package hierarchy (e.g. both 'import
  inveni.config' and 'import config' would work). As a side-effect,
  this would also mask out system-level packages with the same name
  as an Invenio module (e.g. 'invenio.celery' would mask out 'celery'
  module).  (closes #1459)

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>