Pass 'to' through to TaggableRel

When running ```makemigrations``` on Django 1.7, it was crashing with the error ```AttributeError: 'TaggableRel' object has no attribute 'to'```

This commit sends the "to" argument from the TaggableManager init method through to the TaggableRel and stores it as an attribute. This seems to fix the crash I was having.