Reuse Sprite for Normal/Selected/Disabled States in CCMenuItemSprite

[brandmooffin]

This change will also apply to CCMenuItemImage.

If there are no images/sprites specified for selected or disabled states, then the normal image/sprite will be used in its place.

This change can only be ignored if you use the full constructor for either CCMenuItemImage or CCMenuItemSprite.

Example:
public CCMenuItemSprite(CCNode normalSprite, CCNode selectedSprite, CCNode disabledSprite, Action<CCMenuItem> selector)