Create --nomigrations option to disable Django 1.7 migrations

When there are several migrations to run, the database setup can be very slow. This pull request uses [a hack](https://gist.github.com/NotSqrt/5f3c76cd15e40ef62d09) to disable Django 1.7 migrations when the option `--nomigrations` is given.

It should not be a problem when you use `--reuse-db` in your workflow, but the `--nomigrations` flag should help who needs / prefers to create the database on every run.

It seems that [until Django 1.9](https://groups.google.com/forum/#!topic/django-developers/PWPj3etj3-U) is safe to use this hack -- I hope we have another way to do it then. 

There's also a [plugin for manage.py](https://github.com/henriquebastos/django-test-without-migrations) to do it.

My tests took 120 seconds before and are taking just 10 seconds now. So I hope it get merged.