add subscribe_to_all() method to NotificationHandler

- make sip_session.py use it

darcs-hash:20090414104556-b2873-0aefcf3bf27a4dced15dd782dfd24a44c2cc3fad
Updated new Registration object to manage a series of Request objects

darcs-hash:20090414120410-58103-94555d08b89de5e92aa93ef564c8cf734c193a8b