Changed interface of RTPTransport wrt initialization

- No longer starts directly after __init__
- set_INIT() should be called to start initialization
- SCRTPTransportDidInitialize and SCRTPTransportDidFail replace
  SCRTPTransportGotSTUNResponse
- On set_INIT() if STUN request is not needed, SCRTPTransportDidInitialize
  is sent immedeately

darcs-hash:20090220173910-58103-f0fcc52bdc9521f0d04ad5bc03a8f7e69080260a