Rebuilt Subscription object from scratch

darcs-hash:20090525173839-58103-fe4ba78c171bede65dbf8592b85a3e32d3820760
Adapted sip_subscribe_presence.py to new Subscription class

darcs-hash:20090525173920-58103-ecb1d19f4c3e4508a48e3d4deae0dbeb68646527