Fixing a bug in install_as_script, from https://github.com/creationix/nvm/commit/516e553093604ac8a0daa4ad7fab2388cc5c88bf
Install nvm-exec as well when installing via script.

Fixes #553.
Tweaking formatting of help output; adding `nvm exec` example.
Rename `nvm copy-packages` to `nvm reinstall-packages`, and `--copy-packages-from` install option to `--reinstall-packages-from`

For #341. `nvm copy-packages` and install option `--copy-packages-from` will continue to be supported for for at least a full minor release version.
v0.19.0
$NODE_PATH is special, so this function shouldn't risk clobbering it.
Global modules should not be requireable, and `npm root -g` should not be in `$NODE_PATH`.

Fixes #586.
v0.20.0
merge with master