FIX: Set correct main script in package.json

Previously when installed from npm requiring nagios2cfg resulted in the
obscure error, that it couldn't find any file named localhost_nagios2.cfg.

This was due to package.json pointing to test.js which is now relegated
to 
```json
{
    "scripts": { 
        "test": "node test.js" 
    } 
}
```
making it possible to run `npm test`  to validate functionality.