Django 1 7 migrations

This pull request replaces the South migrations with a single Django 1.7 migration.

The question is: what does this mean for people who have existing systems with older Django versions?

* If you have django-fiber 0.13:
  * Your database is up-to-date; there are no new migrations
  * ``python manage.py migrate --fake fiber``
    * see [upgrading-from-south](https://docs.djangoproject.com/en/1.7/topics/migrations/#upgrading-from-south).

* If you have django-fiber < 0.13:
  * Update to django-fiber==0.13
  * Use Django 1.6 or lower
  * ``python manage.py migrate fiber``
    * this applies the south migrations
  * Then, update to the new django-fiber (todo: which version?)

Todo:
* Add description to docs
* Decide what the new version number is (1.0?)