add nvm script for use by other shells

* adds `bin/nvm` for use as a script by any shell
* updates documentation on how to use script
`$NVM_DIR` already contains `.nvm` - you want `$NVM_DIR/current/bin $NVM_DIR/bin`

Also, please note, I'll soon be switching the default behavior of `nvm` to NOT create the "current" symlink.