Upgraded to Jetty 9, updated to new WebSocket API.

Also, we're now exporting a Jetty depend. Most of the time the client doesn't
care, and you have to be major version compatible with what we're using. Since
Jetty 6, 7, 8 and 9 are all in active use, that's easier said than done.
Revamped NexusClient.subscriber to use Subscriber class.

Since you now need a handle via which to unsubscribe, it's not kosher to wrap
up your Callback<FooObject> and throw away the subscription handle that you'd
need to subscribe to it.