Allow filtering by active

* add in a MktFilterBackend which munges 0 into True and False so that we can use nicer non-Python values
that's on the serializer, not on the filter. As noted in the Django Rest Framework docs:

"When using boolean fields, you should use the values True and False in the URL query parameters, rather than 0, 1, true or false. (The allowed boolean values are currently hardwired in Django's NullBooleanSelect implementation.) "

http://www.django-rest-framework.org/api-guide/filtering

I felt it was crappy that we have to put Pythonisms like True and False in the API. If we did that, I could remove all this crap. Think its worth it?