Added send_dtmf() method to Session class

darcs-hash:20090203125007-58103-63bee61699ecbe03521edfd5a14665eb73485657
Respond with same start and stop time in SDP when accepting a remote offer in Session

darcs-hash:20090203134913-58103-b8c83ded2873bcc639a4de2934b5a25c082845cd