Add `system` support to `nvm_print_versions`
If `nvm_ls` is called without a pattern, and node exists, tack on "system".
If nvm is deactivated, display "none" or "system" instead of the system node version
`nvm use system` should work as expected.
Merge pull request #408 from creationix/nvm_system_awareness

Add `system` awareness to `nvm use`, `nvm ls`, and `nvm current`.