discussion: zeit's own view on performant, consistent and semver aware `npm install`

http://zeit.co/ace

main take away's:
- [x] Semver Aware Caching — invalidate cache in semver aware way
- [x] Side-Effects Caching — let say cache postinstall results in node version aware way