Adding `nvm_version_dir`
Use nvm_version_dir in nvm_version_path
Add `nvm_version_greater`
Modify nvm_version_path to use nvm_version_greater.
Making sure that versions are backed up when running tests.
Make `nvm ls` support listing the "versions" directory when it's present.
Make sure `nvm clear-cache` clears the versions directory as well.
Merge pull request #502 from creationix/nvm_version_path

Store versions 0.12.0 and newer in a `versions` subdirectory