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/Makefile

@@ -11,6 +11,9 @@ LIBWEB_OBJS = \
     Bindings/NodeWrapper.o \
     Bindings/WindowObject.o \
     Bindings/Wrappable.o \
+    Bindings/XMLHttpRequestConstructor.o \
+    Bindings/XMLHttpRequestPrototype.o \
+    Bindings/XMLHttpRequestWrapper.o \
     CSS/DefaultStyleSheetSource.o \
     CSS/PropertyID.o \
     CSS/Selector.o \
@@ -54,6 +57,7 @@ LIBWEB_OBJS = \
     DOM/ParentNode.o \
     DOM/Text.o \
     DOM/Window.o \
+    DOM/XMLHttpRequest.o \
     StylePropertiesModel.o \
     DOMTreeModel.o \
     Dump.o \