Bugfix: Added private static $defaultFormat...

... in order to choose default via config. Permit WCAG validation of XHTML (http://achecker.ca/checker/index.php).

Achecker validator's request are really basic:

```
GET / HTTP/1.0
Host: cmondovi31.zirak.it
```

We can't check user agent like we do for W3C validator, so we need to define a customizable default.