swift引入后，为了使oc和swift更相近，对oc的初始化方法也进行了修正，具体说明，见下面的链接，这个waring的最简单的修正方法是，到相应类的头文件中，去掉在自定义初始化方法后面的 NS_DESIGNATED_INITIALIZER宏。这样系统就不会认为我们定义了designated initializer，仅仅定义了一个convience initializer，就不会报错了！
重要的解释如下：
The designated initializer guarantees the object is fully initialised by sending an initialization message to the superclass. The implementation detail becomes important to a user of the class when they subclass it. The rules for designated initializers in detail:
- A designated initializer must call (via super) a designated initializer of the superclass. Where NSObject is the superclass this is just [super init].
- Any convenience initializer must call another initializer in the class - which eventually leads to a designated initializer.
- A class with designated initializers must implement all of the designated initializers of the superclass.
详细解释：