Added nodejs_version and nodejs_packages variables

- 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
- Added a check to see if there are any packages given to be installed