InvenioConnector: mechanize dependency
InvenioConnector: mechanize dependency

* python-mechanize package is an optional Invenio dependency.
  However _SGMLParserFactory and CDSInvenioConnector classes
  in invenio_connector.py were assuming its existence.
  This is now fixed.

Reported-by: Genis Musulmanbekov <57eed2248fcc0b2872e5d7992e1d55a734b09437@jinr.ru>
Reviewed-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio