Add node.js executable for starting browser with a participant

[jonnor]

To fit into the component library model of MsgFlo. And by extension also fbp-spec.

Ideally an embedded browser, but could initially be opening system browser.
Since user interfaces is an important usecase, should probably be a regular browser UI.
Headless (like phantomJS) can be an option later, though it may be useful for automated tests. See #1