added hue.discover (uses UDP broadcast to find bridge)

This is ported from ruhue, a Ruby HUE library I tinkered with a few months ago (I'm happy to be able to switch back to node).  At the moment I don't need the registration code, but that's pretty simple if you want to add it yourself:

https://github.com/Burgestrand/ruhue/blob/master/lib/ruhue/client.rb#L34

Also added example.js, based on your example in the README, but using discover to get the host and totally random colors to make the result more obvious.