Initializing migration from django-piston to django-restframework (bug 974952)

Moving `api.views` to `api.views_drf` while migrating piston to DRF.

See [bug 974952](https://bugzilla.mozilla.org/show_bug.cgi?id=974952) for further details.
This is part 1: moving `AddonDetailView` and `LanguageView`.
The ticket is being splitted.

Latests tests results are here: https://ci-addons.allizom.org/job/marketplace-build-branch/31/ (passing)