javascript的继承
来自 CasualJS，参考JavaScript高级程序设计(第2版)
//===================================================

/**
 * Inheritance implementation for Javascript.
 */
var inherit = function(childClass, parentClass) 
{
	var tmpConstructor = function() {};
  	tmpConstructor.prototype = parentClass.prototype;
  	childClass.superClass = parentClass.prototype;
  	childClass.prototype = new tmpConstructor();
  	childClass.prototype.constructor = childClass;
        return  childClass;
};