Renamed MediaTransport to RTpPTransport, added AudioTransport class and (temporarily) hacked it into the current design

darcs-hash:20081111125129-58103-037509fe59dedaba36f475c70cefc49eac552328
Added functionality to sip_rtp_audio_session to change echo cancellation tail length on the fly with "<" and ">" keys

darcs-hash:20081111143345-58103-7ff9cc97644849a81a8c562adf7ff1c70d4654d9