Implementation dependant parameters, such as message type or quality,
must be defined by the gateway class, not in the common
ShortMessage_send script.

This should make it easier to switch implementations.

Also:
 * 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
   message_id_list
 * Split interfaces in two SMS Sending / SMS Receiving.