add websocket notifications of public content

fixes #27

The basic structure here is based on the same twsock setup used
in tiddlyspace and the tiddlersocket.js code from
http://tapas.tiddlyspace.com/

The difference here is that pretty much every page gets a notification
icon that you can click to see latest happenings. The icon will change
when something new comes in.

Getting this deployed requires adding tiddlywebplugins.dispatcher
and tiddlywebplugins.jsondispatcher and the nodejs-based twsock

Managing those services raises the bar on admin but c'est la vie.