Travis envs for Django 1.7

Couple fixes for test runner compatibility
ForeignKeys for content-type relation in djangoorm storage

Makes Django 1.7 checks framework happy and allows reverse
relationships from conenttype model
Migrations for Django 1.7 and South
Ignore South migrations in flake8
Two migrations for Django 1.7
Merge pull request #13 from coagulant/feature/django1.7

Django 1.7 compatibility
updated docs
Merge branch 'develop'
Bump version: 0.0.21 → 0.0.22