Implement a media type "proxy" resource

[bruth]

There are subtle differences between representations for a resource, namely an HTML vs. JSON representation. An HTML representation may contain implementation specific components while a JSON representation may be agnostic to a particular implementation.

This proxy resource would act as the default, but would check for registered resources that provide the requested representation.

# hypothetically..
class Resource(ProxyResource):
    media_types = {
        'application/json': JSONResource,
        'text/html': HTMLResource,
    }