invenio-backup-site: initial release

* Initial release of invenio-backup-site and invenio-restore-site
  devscripts.  Useful e.g. to quickly restore previously-installed
  and previously-backed-up vanilla demo site without having to run
  potentially lengthy invenio-recreate-demo-site devscript again.
  (which would BTW also necessitate to be fully online)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>