Initial package release

* Creates invenio_demosite package. Until now, invenio_demosite  was
  being developed within the Invenio project main source code base.
  See <http://invenio-software.org/> for more information.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Co-authored-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Co-authored-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>