Enhanced xcapdiff subscription process

Ignore-this: 160071911b51418b520339b745304631
    - Take into account the Min-Exires header in case of error 423
    - Don't retry infinitely if the error received is 405, 406 or 489
    - Stop listening for notifications after unsubscribe

darcs-hash:20101007103947-56b4d-7398bffe6376d5cf5fc30744216e00b5b7b6c8fd
Added PositiveInteger datatype

Ignore-this: 7a872dcec28babca6d74e6b5a188c41d

darcs-hash:20101008033155-b22ec-06e8cb7f59677403257762765e7b02d302a27cee