Eliminate build step for node js and mocha

I used the [UglifyJS2 trick](https://github.com/mishoo/UglifyJS2/blob/master/tools/node.js) to use the real source files in node.js and made regular `mocha` work. This makes the lib nicer to work with when `npm link`ed.

IMO the next step should be removing `lib/*` from version control. Wouldn't it work if those were in `.gitignore` and only got built right before publishing a new package to the npm repo?