Presence : Enabling of presence subscription

This code enable presence subscription / notification based on Asterisk hint mechanism.
The presence of a SIP account will be automatically pushed by Asterisk server.
DBus interface in CallManager (subsribePresence) is added to subscribe the presence of a buddy uri for an accountID.
Then presence notification are received in buddysip.cpp. Callback to UI must be there implemented.
Lot of commented code is still in the code. Work must be done to clean it.