Add package_versions attribute

The `install_from_package` recipe doesn't provide a way to specify versions. Since versions for distro packages will be different from versions of node.js itself, this attribute is a hash of distro package name => distro package version. You need to know what distro packages you will be installing.

If you don't set anything, then the package resource will just get `version nil` and default to the latest version as it does now.

Example usage:

```
node.set['nodejs']['package_versions']['nodejs'] = value_for_platform('ubuntu' => {
    '10.04' => '0.10.21-1chl1~lucid1',
    '12.04' => '0.10.21-1chl1~precise1',
  })

include_recipe 'nodejs::install_from_package'
```