PubSub implementation

[jfyne]

This PR allows multiple instances to communicate via Broadcast.

• PubSub object which allows handlers to communicate with each other
• PubSubTransportdescribes how the PubSub should send messages.
• LocalTransport enables in process handler communication

See this for a more complex example https://github.com/jfyne/live-examples/pull/4

[jfyne]

@dstpierre this is my first pass at what we discussed. There is a new PubSub primitive (which I might change the name of) and a PubSubTransport interface so that you can bring your own message broker. Included is a LocalTransport which can enable in process communication between handlers.

There is an example linked that shows how to use the Go CDK to achieve multi node broadcasting. It uses an in memory broker in the example, but it would be trivial to change it to one of the many brokers the Go CDK supports.

I'm going to keep playing with this, before I merge it.