handling rejected m-lines

[fippo]

rejected m-lines are currently not handled properly (and it's not completly clear what the spec on this is). See http://www.ietf.org/mail-archive/web/stox/current/msg00318.html

An easy way to do this would be to translate a port 0 to a content without a transport element -- which might fit chromes behaviour of not adding ice candidates for rejected m-lines quite well.