Add "audit" mode, which records HTTP code and download time.

Thanks for making this and open sourcing it.

I've made this small change because I wanted to audit my sitemaps, checking the HTTP codes they return and the time to download a page.

Sample output:
```
200     733.79  http://www.example.com/url1
200     1048.94 http://www.example.com/url2
200     824.75  http://www.example.com/url3
200     796.07  http://www.example.com/url4
200     960.86  http://www.example.com/url5
404     655.77  http://www.example.com/url6
200     1016.34 http://www.example.com/url7
200     755.66  http://www.example.com/url8
500     654.50  http://www.example.com/url9
```

I then use this to look for non `200` responses, find pages that are inefficient to generate, and also to audit the contents of my rather large sitemap.