Fixed bug when updating sound devices on ConferenceBridge to None/None

darcs-hash:20090703161154-58103-f6fae9ca5ceef88da8167a353376973768e2cce1
Added stop() method to sipsimple.core.ToneGenerator

darcs-hash:20090706111322-58103-3dbe29f7013f365da80156756affd133392cfbce