Remove the need for a settings_local.py (bug 1050155)

fixes [bug 1050155](https://bugzilla.mozilla.org/show_bug.cgi?id=1050155)

This means that the `settings_local.py` won't be used automagically by `manage.py` anymore. If you want to use it, either:
- rename it to `local_settings.py` (imported, if it exists, by the new `settings.py` file), and make sure that you change the first line to be `from settings import *` (don't import from `lib/settings_base.py` or from `default.settings` directly, I'm not sure what would happen)
- use `DJANGO_SETTINGS_MODULE=settings_local` or the `--settings=settings_local` parameter when you run commands.

I've checked with @jasonthomas, this should be ok with his last changes (can you please give a look at this PR and confirm that it looks ok to you?)
They aren't for now, but might be again in the future... we'll have to discuss that, but in another PR/bug.