Set default representer config

Here is a callback method in `ControllerAdditions` allowing to send default url options to the representer config from the controller's `request`object.

This avoids having to set these options in environment config files when you want to use the current request host, port and protocol.

However, these options can still be overridden in environment config files as before (static config is merged over default one).

The `set_representer_config` method could be used to set other representer default configuration values from the controller (for now, only url options seems useful).

If you aggree with this feature, I can update the readme and/or specs.