Cache npm_modules and bower_components

This PR closes #8.

It does several things:

- Caches `npm_modules` between deploys like the [Node.js Buildpack](https://github.com/heroku/heroku-buildpack-nodejs)
- Caches `bower_components` between deploys
- Removes hardcoded node version in favor of resolving to the correct version based on `package.json` and https://semver.io
- Updates the readme with how to clear the cache as you see fit