Provide class based view support in decorator

Based on
[this](https://github.com/FelixSchwarz/django-twilio/commit/f169ea26ce13
f24bd78fa2a3cb7f91dafaf28c9b) commit which was never made into a pul
request.

Solves #22
Merge pull request #33 from rdegges/class-based-view-support

Provide class based view support in decorator, solves #22