Use django rest framework 3.0 (pre-release)

See
https://github.com/tomchristie/django-rest-framework/blob/version-3.0/docs/topics/3.0-announcement

- Point requirements.txt at a specific commit.

- Replace `lookup_field` with `extra_kwargs` in `HyperlinkedModelSerializer` for location, see https://github.com/tomchristie/django-rest-framework/blob/version-3.0/docs/topics/3.0-announcement.md#changes-to-hyperlinkedmodelserializer

- Use `.query_params` rather than `.QUERY_PARAMS`, see https://github.com/tomchristie/django-rest-framework/blob/version-3.0/docs/topics/3.0-announcement.md#the-data-and-query_params-properties

- Remove `method_name` from `SerializerMethodField`, see
https://github.com/tomchristie/django-rest-framework/blob/version-3.0/docs/topics/3.0-announcement.md#optional-argument-to-serializermethodfield