Implement database functions for Alternate Gateways

Added nongammu_model.php as a superclass for all Alternative Gateways.
Implemented:
 - a stub function for sending message (really_send_massages())
 - saving a message to sentitems after sending
 - saving a message into outbox for to-be-postponed messages
 - processing of postponed messages from outbox
 - crontab script for periodic executing