Rework settings machinery

* [x] Find a nice way to overide django:settings from *etc/settings.ini*
* [ ] Investigate why ``TEMPLATE_DIRS`` needs explicitly *mapentity/templates* (since we have eggloader)
* [ ] Move ``MEDIA_ROOT`` and ``STATIC_ROOT`` to *etc/settings.ini* for DRY between Django settings and buildout ?
* [x] Use ``DEPLOY_ROOT`` in buildout instead of ``buildout:dir``