Adde ajaxFormSubmit to submit forms via AJAX.

Hi! I'd like to add the support for form submitting via AJAX from JSP/JSF. The current version works correctly if we put ajaxFormSubmit in the HTML form like this:

```
onsubmit="${wicket:ajaxFormSubmit('POST')}"
```

Let me know if you like it or not :)

@klopfdreh