Move creation of call from callback to -makeCallTo: method
Separate code in call state change callback

PJSIP-related code is separated from AKSIPUserAgent-related
code. Because the callback can be called on any thread,
AKSIPUserAgent-related code is serialized on the main queue. This is
now done in one big block to be easier to understand.
Add printing log of call states
Log incoming call right away
Log call media stats on the level 4
Implement a callback for call replacement

Create a new call object from the callback.
Hang up call on successful transfer
Separate code in media state callback

PJSIP code is separated from AKSIPUserAgent code. AKSIPUserAgent code
is called in one block on the main thread.
Rephrased log file
Remove unneeded autorelease pool