compatibilty with the node.js buildpack for installing node

This adds special support for using `package.json` for installing a specific version of the node when using [multibuildpack](https://github.com/ddollar/heroku-buildpack-multi) and the [node.js buildpack](https://github.com/heroku/heroku-buildpack-nodejs). So you can now add the following to your `package.json`:

```
  "engines": {
          "node": "0.10.x"
  }
```

/cc @rjackson