Added "node_version" and "node_packages" to Vagrantfile

- You can change the default Nodejs version
- Now you can use NVM in zsh as well.
- Uses custom nvm_install.sh @ https://gist.github.com/Ilyes512/8335484
- Global Node Modules/Packages as mentioned at #34 are now in node_packages.sh
- Deleted yeoman.sh as it is unnecessary now
Merge pull request #84 from Ilyes512/improve_nodejs

Added "node_version" and "node_packages" to Vagrantfile