Add a Javascript Console

This adds a javascript console to nwm.

I have added a keyboard shortcut `Meta + Shift + Escape`
that opens the console inside the windows manager.

Also, there is a script to open the console outside the windows manager,
which is useful when running in xephyr.

Finally, I've added documentation for this in the readme.

I don't make this pull request so much because I think you should pull it right now,
obviously things are in flux right now. but more to show you what I am doing and 
show the watchers that stuff is happening!

So I can connect a console and move windows around arbitrarily. 
needs a binding to z-index or whatever it's called to get control of depth.

I'm gonna look into this next, but I don't really know C, so who knows what will happen. 
@trans if you can help point me right direction I'd be greatful!

I'm dominictarr on irc. I hangout in #node.js on freenode a lot.