Fix refcounting of the JingleSession
Fix refcounting of the JingleSession

When asking the jingle factory to create a session it gives you back a object
you don't have a ref to, so first of all make sure we have a ref. And second,
do actually unref it when disposing the channel :)

mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble