[WebProfilerBundle] Show AJAX requests in the symfony profiler toolbar

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Adds AJAX requests in the web debug toolbar.
See #8896 for the original discussion.

![image](https://cloud.githubusercontent.com/assets/47313/4384087/43d1feb2-43b0-11e4-99c9-3e50e19e623f.png)

```js
var infoSpan = document.getElementsByClassName("sf-toolbar-ajax-info")[0];

var children = Array.prototype.slice.call(tbody.children);
for (var i = 0; i < children.length; i++) {
    tbody.removeChild(children[i]);
}
tbody.appendChild(rows);

if (infoSpan) {
    var text = firstItem == 0 ? requestStack.length + ' calls' : 'Last 20 calls out of ' + requestStack.length;
    nfoSpan.textContent = text;
}
```