Take -version from package.json

E.g.

``` js
program.version(JSON.parse(require('fs').readFileSync(
     require.main.filename.match(/^(.+)\/.+$/)[1] + '/../package.json')).version)
```

But a test is needed for the existence of package.json, I think the 2 most used locations are <package root> and <package root>/<some dir>.

So 1, check for `main_module_dir/package.json` and 2, check for `main_module_dir/../package.json`.
