Added a function to the javascript Date object (via prototype)

Extends javascript Date object with a format function, wich will call
jQuery.format.date with itself as value and a given format.

```javascript
var d = new Date();
document.write($.format.date(d, "dd/MM/yyyy"));
document.write(d.format("dd/MM/yyyy")); //this wil do the same as the line before
```

(EDIT: maybe i put the Date.prototype-code at the wrong place, i'm not that familiar with jQuery-plugin-development.. :P)