Description:
    An iOS like button. When pressed, the button will become gray.

Purpose:
    When add a Button, we need write a selector for it to set
    diffenent background for each status (pressed, focused,
            normal...). 
    But most time, we only need a button with normal and pressed
    status, write a selector is too heavy. So XButton comes, you
    only need set the normal status's background, when pressed it
    will become gray to show users that "you pressed me".

How to use:
    Just replace the widget Button by me.maxwin.view.XButton
    When you need to change the background, text, call method
    reset() to clean the cached highlight bitmap please.