code style tweaks

Mainly just style tweaks to your version, I think I'm cool with the rest of your code on this branch. Feel free to merge this this to rvagg/node-levelup/master if you're happy with the state of it and I'll cut a new version.

(I also found the `rulers` setting in Sublime Text 2 so I'm trying to stick to 80 chars line length in this file, not that it really matters or that I normally care but so far the style has been aiming for roughly that.)