fix memory leak when use menu_selector.

1.MenuItemFont::create("reset", this, menu_selector(UpdateLayer::reset ));
2.MenuItemFont::create("reset", CC_CALLBACK_1(UpdateLayer::reset, this));

The first style will cause memory leak,
the menuitem(child) owns the parent("this")'s reference,
the parent(this) owns the menuitem's reference.
At last they both can not release.

we can not release the target's reference in the menuitem's deconstructor.
or we do not retain the target when initialize.