Issue 165: Ice re-gathering

Currently the text on RTCIceTransport.start states:

"The first time start is called candidate connectivity checks are started and the ICE transport attempts to connect to the remote RTCIceTransport. If start is called with invalid parameters, throw an InvalidParameters exception. If start is called again, an ICE restart occurs, all remote candidates are flushed, and addRemoteCandidates or setRemoteCandidates must be called to add the remote candidates back or replace them."

In an ICE restart as defined in RFC 5245, the ICE ufrag/password is changed and candidates are re-gathered.  This would imply constructing a new RTCIceGatherer object to use when calling RTCIceTransport.start again.  But what if it is desired to restart ICE connectivity checks with a new set of gathered candidates (possibly with a different gather policy) without having to change the ufrag/password?

In this situation it would be useful to add a gather method to the RTCIceGatherer, as follows:

partial interface RTCIceGatherer {
    void gather (optional RTCIceGatherOptions options);
};