Merge pull request #501 from creationix/fix_copy_packages

Fix nvm copy-packages
Adding `nvm_version_dir`
Use nvm_version_dir in nvm_version_path
Create nvm_version_path
Use nvm_version_path everywhere possible.
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.
Should escape grep in copy-packages.
Merge pull request #503 from egilkh/master

Should escape grep in copy-packages.
Merge pull request #502 from creationix/nvm_version_path

Store versions 0.12.0 and newer in a `versions` subdirectory
Adding unit tests for nvm_ls_current. Relates to #487.
Fixes #487.
Fixed `nvm_version_greater` has syntax error in zsh-buildin-command `[`
Fix tests so that they fail in zsh (for #505)
Merge pull request #505 from magicdrive/fix-error_nvm_version_greater

Fixed `nvm_version_greater` has syntax error in zsh-builtin command `[`
Make sure the new `versions` directory is filtered out of nvm_ls output (in zsh).
Fixing nvm_version_greater tests.

Relates to #505, #506.