Fix #176. Update to the latest IceServer definition

Old:

dictionary RTCIceServer {
    DOMString  url;
    DOMString? credential;
};

New:

dictionary RTCIceServer {
    (DOMString or sequence<DOMString>) urls;
    DOMString?                         username = null;
    DOMString?                         credential;
};

Note: Current Firefox Nightly "28.0a1 (2013-11-18)" Does not support multiple 'urls'