Validate an API plugin, ensuring it is an API plugin and has the
        necessary fields present.

        `plugin` is a subclass of scruffy's Plugin class.

def valid_api_plugin(self, plugin):
        """
        Validate an API plugin, ensuring it is an API plugin and has the
        necessary fields present.

        `plugin` is a subclass of scruffy's Plugin class.
        """
        if (issubclass(plugin, APIPlugin)       and
            hasattr(plugin, 'plugin_type')      and plugin.plugin_type == 'api' and
            hasattr(plugin, 'request')          and plugin.request != None and
            hasattr(plugin, 'request_class')    and plugin.request_class != None and
            hasattr(plugin, 'response_class')   and plugin.response_class != None):
            return True
        return False