LibWeb: Add XMLHttpRequest object :^)

This patch adds very basic XMLHttpRequest support to LibWeb. Here's an
example that currently works:

    var callback = function() { alert(this.responseText); }
    var xhr = new XMLHttpRequest();
    xhr.addEventListener("load", callback);
    xhr.open("GET", "http://serenityos.org/~kling/test/example.txt");
    xhr.send();

There are many limitations and bugs, but it's pretty dang awesome that
we have XHR. :^)

Libraries/LibWeb/Forward.h

@@ -51,6 +51,7 @@ class StyleResolver;
 class StyleRule;
 class StyleSheet;
 class Window;
+class XMLHttpRequest;
 
 namespace Bindings {
 
@@ -66,8 +67,10 @@ class NodeWrapper;
 class WindowObject;
 class Wrappable;
 class Wrapper;
+class XMLHttpRequestConstructor;
+class XMLHttpRequestPrototype;
+class XMLHttpRequestWrapper;
 
 }
 
-
 }