Added IncomingSubscription class to sipsimple.core

darcs-hash:20090717162902-58103-f43650f700fb89386df0c197ad73ae5d4cc8dd61
Also protect ToneGenerator.play_tones() against overflow exception

darcs-hash:20090717163828-58103-22bff2396d21b61e0f933f988a2961a530d10385