function loadJson(path, cb) {
	var xobj = new XMLHttpRequest();
	xobj.onreadystatechange = function () {
		if (~~xobj.readyState !== 4) return;
		if (~~xobj.status !== 200) return cb('xhrError:' + xobj.status);
		return cb && cb(null, JSON.parse(xobj.response));
	};
	xobj.open('GET', path, true);
	xobj.send();
}
@function module:loader.loadJson
@desc     load a json file

@param {String}   path - file path
@param {Function} cb   - asynchronous callback function