Add "nodejs_version" to adjust the default nodejs version
NVM now also works when using ZSH

Cleaned up nodejs.sh and created a new nvm install.sh that removes unnecessary things and adds nvm to both ~/.bash_profile and ~/.zshrc. If the last 2 dot files don't exist they will be created.
Installing Global Node Packages now happens within nodes.sh
Deleted yeoman.sh
Added more info on NVM in readme
Switched "yeoman" Node Package with "yo"