Reguster a plugin for the model.

        The only one plugin can be registered. If you want to combine plugins, use CompoundPlugin.

def register_plugin(cls, model, plugin):
        '''
        Reguster a plugin for the model.

        The only one plugin can be registered. If you want to combine plugins, use CompoundPlugin.
        '''
        logger.info("Plugin registered for %s: %s", model, plugin)
        cls.plugins[model] = plugin