Tweak and simplify Node.js/Grunt buildpack.

Tweak and simplify based on the following assumptions:
- package.json, etc. live in BUILD_DIR/frontend
- Compilation script needs to run `bower install`
- Don't need to augment package.json with grunt and grunt-cli
- Don't need to generate Procfile
- Don't need Compass
- Don't need to modify PATH via profile.sh scripts
- Don't need to report to the mysterious nomnom service