Add webhook support

Initial support for webhooks.  Implementation is built to support additional events and event handlers in future.  Currently only one event is being raised - a DocumentCommitEvent, after a doc is successfully updated in updateDoc.

EventManager maintains a channel (eventChannel) for raised events.  The only synchronous activity on the main document update go routine is to add the event to the EventManager channel.

EventManager has a go routine that works the eventChannel.  For each event, it passes the event to each registered eventHandler.  Each eventHandler creates a go routine for their own processing (for webhook, it's making an HTTP POST).

eventHandlers are defined in config.  Currently the only supported eventHandler type is webhook.