Adding a "designSize" property to CCDirector for the fixed size mode.

Directly related to the discussion in https://github.com/cocos2d/cocos2d-iphone/issues/413 and https://github.com/cocos2d/cocos2d-iphone/pull/424.

Basically it adds a user definable CCDirector.designSize property that defaults to the value of CCDirector.viewSize. If we add this I really think we should make CCDirector.viewSizeInPixels private so there aren't so many methods that do nearly the same thing.