Register an emitter.

        Parameters::
         - `name`: The name of the emitter ('json', 'xml', 'yaml', ...)
         - `klass`: The emitter class.
         - `content_type`: The content type to serve response as.

def register(cls, name, klass, content_type='text/plain'):
        """
        Register an emitter.

        Parameters::
         - `name`: The name of the emitter ('json', 'xml', 'yaml', ...)
         - `klass`: The emitter class.
         - `content_type`: The content type to serve response as.
        """
        cls.EMITTERS[name] = (klass, content_type)