[TIMOB-95] Override built-in XMLHttpRequest to use Titanium.Network

To make it seamless to support libraries such as jQuery, Mootools, YUI, etc that have their own AJAX convenience methods, we need to patch XMLHttpRequest to use Titanium.Network.createHttpClient

such as:

window.XMLHttpRequest = function() {
    return new Titanium.Network.createHTTPClient()
};

test case w/ jquery:

$.getJSON("http://api.flickr.com/services/feeds/photos_public.gne?tags=cat&;tagmode=any&format=json&jsoncallback=?",
                function(data)
{
         $.each(data.items, function(i,item){
        $("<img/>").attr("src", item.media.m).appendTo("#log");         
            if ( i == 3 ) return false;
        });
});

(make sure you create a div with id log in HTML)