Avoid calling "nvm_ls current" and call "nvm_ls_current" directly
Adding a slow `nvm current` test, since it needs an installed version of node.
v0.11.2
Add `system` support to `nvm_print_versions`
If `nvm_ls` is called without a pattern, and node exists, tack on "system".
Add nvm_tree_contains_path function
If nvm is deactivated, display "none" or "system" instead of the system node version
`nvm use system` should work as expected.