Support for custom Authorization header scheme name

Currently, the expected scheme name in the Authorization header is `Bearer`. There doesn't seem to be any consensus on what it should be as it is not specified in the IETF draft. Since `Bearer` conflicts with OAuth2, `JWT` is preferable (plus, in the WWW-Authenticate header, the auth scheme is "JWT"). At least giving the user the option to configure this is desirable.

This patch is modelled after the [Django REST framework JWT Auth](https://github.com/GetBlimp/django-rest-framework-jwt#jwt_auth_header_prefix).