JSONRPC 2.0 Notification support

With rpc2 bi-directional support, I really want it to support json notification,

The specification[1] says notification was from client to server, fire and forget. I suppose we could think server was kind of jsonrpc client too, so it can send notification to the connected clients(something like pubsub).

I create this pull request for review, course I'm new to golang and rpc, do not known if I'm doing it wrong or not following the best practice.

[1] http://www.jsonrpc.org/specification#notification