Change configuration so that there's only one node_modules at root level instead of one per package

Normally should just change `.build/postinstall.js` so that we use 
[yarn install --modules-folder <path>](https://yarnpkg.com/lang/en/docs/cli/install/#toc-yarn-install-modules-folder) to point to a common folder to all packages.

Another option is to create symlinks in Linux/OSX or a [junction in windows ](https://msdn.microsoft.com/fr-fr/library/windows/desktop/aa365006(v=vs.85).aspx).

