[tox] Enable all applicable py34 environments
All except etwist.
Comes with a few tricks in tox.ini:
* add ``-e.`` to force installing of cubicweb in development because
dependency cubes would otherwise install cubicweb 3.22 (the latest release),
which is not python3 installable.
* "install" cubes/__init__.py because the file is not installed in
develop mode; maybe this file should be managed by cubes themselves...