installation: SQLAlchemy, SQLAlchemy-Utils upgrade

* Updates minimum and maximum version requirements for
  SQLAlchemy and SQLAlchemy-Utils.  (closes #1776)

* Fixes compatibility problems with SQLAlchemy>=0.9 version.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
installation: addition of OS X installation guide

* Addition of a section on how to install invenio software
  on OS X 10.9 or later.  (PR #2392)

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
general: importing modules from packages fix

* Uses Flask-Registry for importing modules from packages

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>