Gitless in Seattle 

Few changes to @ronkorving 's contribution:

1. We do now blow up the directory hosting the installed node versions and global modules
2. We update only nvm.sh, which is faster to download and does not require tar.
3. Just like my prior pull request, we base ourselves on NVM_DIR instead of NVM_TARGET, which means we will detect (and update) the correct installation (instead of potentially double-installing)