[1'b68'1 astrad V8] tester et documenter les options (min, max, defaut) expiry

[Task-by Oussama Hammami <oussama.hammami@switzernet.com>]

- Tester ces options avec le module RT (mysql).
- Documenter ces testes.

;maxexpiry=3600                 ; Maximum allowed time of incoming registrations
                                ; and subscriptions (seconds)
;minexpiry=60                   ; Minimum length of registrations/subscriptions (default 60)
;defaultexpiry=120              ; Default length of incoming/outgoing registration

http://www.asterisk.org/doxygen/trunk/Config_sip.html