[Galene] Protocol changes

[Juliusz Chroboczek <jch@irif.fr>]

I'm preparing Galène for simulcast, and that has caused some changes to
the protocol.  In short, tracks are no longer labelled, we're labelling
streams, and we now use the track.kind and track.rid fields to
disambiguate.

This has some consquences both on the protocol and the protocol.js
interface.  At the protocol.js level:

  * there is no longer a c.labels dictionary;
  * there is a new c.label field;
  * there is no longer a c.kind field, since c.label subsumes it;
  * the syntax of the sc.request method has changed.

I've added a new sc.users dictionary, that contains some information about
all the users in the group:

  - username;
  - permissions (op, etc.);
  - status (raised hand, etc.);
  - published streams.

Alain is planning to use this information in order to make the users' list
more informative.

I've also switched to a more recent API (addTransceiver instead of
addTrack); however, this should not break anything on reasonably recent
browsers.

-- Juliusz