nvm exec wrapper to execute commands with a specific node version

I've created a very basic wrapper for sourcing nvm and setting a node version with `nvm use` to easily run a command with a specific node version.

```sh
NODE_VERSION=0.10 ~/.nvm/nvm-exec node -V
```

I'm currently using this as a solution for using [nvm in capistrano deployment](https://github.com/koenpunt/capistrano-nvm).

Suggestions for improvements are welcome.