general: Elasticsearch logging with Lumberjack.

* Added disabled-by-default config options for Elasticsearch logging.

* When Elasticsearch logging is configured, don't populate rnkPAGEVIEWS and
  rnkDOWNLOADS.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
Merge branch 'elasticsearch_logging' of https://github.com/jmacmahon/invenio into prod

Conflicts:
	requirements.txt