installation: dependency links renovation

* Prepares `setup.py` for Invenio v2.0 release for pip<1.6.
  (addresses #1797)

* NOTE use `pip install --process-dependency-links -e .`

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
formatter: fix Bibfmt model import

* Fixes broken import of `Bibfmt` model.  (closes #1781)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>