Support relay chaining

[gavllew]

Migrating https://code.google.com/p/crocodile-msrp/issues/detail?id=1

As explained in draft-pd-msrp-websocket, the WebSocket server must act as an MSRP relay for the Javascript client. However, RFC 4976 allows for operating through an arbitrary number of relays; currently this is not supported.

It should not be particularly difficult - just needs further authentication steps before the SDP exchange, and tracking of authentication expiry for each of the relays. However, we may want a different set of relays for each session (based on the destination), and we may need to allow different credentials for each relay.