Provide South >=1.0 migrations, so the app can work with django < 1.7

@ivanvenosdel Can you try this ?

I followed https://docs.djangoproject.com/en/1.7/topics/migrations/#libraries-third-party-apps to provide backward-compatible migrations.

As noted in http://south.readthedocs.org/en/latest/releasenotes/1.0.html#library-migration-path, South >= 1.0 is required for this.