Neater build (separate compiled files)

I figured I'd remove the compiled JS files from the repo.

This way, the compiled files live under `./lib` where they can easily be git-ignored and to where they can be generated on the npm `prepublish` script. (It also runs on `npm install`. I followed [npm recommendations](https://www.npmjs.org/doc/misc/npm-scripts.html).)

I also added a `files` list to `package.json`, so npm only packages what's necessary (i.e. only the compiled files and readme).