Use django.utils.six instead of compat.py

Fixes this with_metaclass() error when using django 1.7:

      File "/Users/cdestigter/checkout/django-typed-models/typedmodels/__init__.py", line 2, in <module>
        from .models import TypedModel
      File "/Users/cdestigter/checkout/django-typed-models/typedmodels/models.py", line 261, in <module>
        class TypedModel(with_metaclass(TypedModelMetaclass, models.Model)):
      File "/Users/cdestigter/checkout/django-typed-models/typedmodels/compat.py", line 12, in with_metaclass
        return meta("NewBase", bases, {})
      File "/Users/cdestigter/checkout/django-typed-models/typedmodels/models.py", line 44, in __new__
        meta, classname, bases, classdict)
      File "/Users/cdestigter/checkout/django/django/db/models/base.py", line 72, in __new__
        module = attrs.pop('__module__')

re #16
Run tests on django 1.7 #16