Add support for Django REST Framework

[barseghyanartur]

The class DefaultDjangoContextBuilder of the error_tracker.django.utils needs some extensions. In particular, in case of Django REST Framework (DRF), the request instance has no POST (request.POST is empty dict). Instead data is stored in request.data. You could simply check, if it's instance of rest_framework.request.Request, then handle as DRF request, otherwise standard Django.