Support node-style callbacks

Hey! I started playing with Bacon after attending your workshop at mloc.js. I really like it so far. Here's one addition I'd find useful: support for Node.js-style callbacks. This makes bacon.js much easier to use on the server side.

I realize that this is a bit like the jQuery adapter so maybe it should be in a separate package, but the readme explicitly mentions that bacon.js works fine on the server; this looks like a basic requirement for node.js usage.

Hope you like the idea; please give feedback on the implementation if you'd prefer some other solution :)

Keep up the great work! :+1: