Implementation dependant parameters, such as message type or quality,
must be defined by the gateway class, not in the common
This should make it easier to switch implementations.
* send method is expected to be called once per recipient. I guess we
prefer to isolate messages than to batch send messages efficiently.
* send method expect relative url of sender and reciever documents, so
that it can decide which properties to use.
* drop `sender_title` parameter, now the gateway implementation decide
what to use.
* drop `test` parameter. For simulation mode, set simulation mode on
the gateway instance
* sms.send() no longer accept direct from_url / to_url / body message.
It only supports sending what's defined on document properties.
* SMSTool_afterSend will be called with message_id parameter, not
* Split interfaces in two SMS Sending / SMS Receiving.