[callchannel] Don't play tone if MT call disconnects before answering

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>
Merge pull request #15 from marttipiirainen/tone_mtdrop

[telepathy-ring] Don't play tone if MT call disconnects before answering
Fixed disconnect of IncomingMessage and ImmediateMessage signals
Merge pull request #16 from monich/devel

[telepathy-ring] Disconnect IncomingMessage and ImmediateMessage signals