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 <genis@jinr.ru>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>