Fixed screen size mode.

Basically done. Still not sure what to do about the viewSize problem mentioned in #413 though. If a dev is using the fixed size mode, there is no way of getting the correct screen size other than hard coding it. Maybe I should just expose the FIXED_SIZE variable and they just need to know to use that instead of CCDirector.viewSize? The default sizes of CCScenes and a few other nodes will be wrong too.

Also, it searches for the smallest power of two that will cover the screen. I'm not convinced this is the best solution, since it will pick a 4x content scale on a 1280x720 Android tablet and cut off a *lot* of content (1/3 of the width and over half the height). Either the dev needs to ship the game with 3x art (terrible option), or we need to add some letterboxing to avoid severe cropping like this.

I did change some of the enums to NSStrings since this is intended to be used with an NSDictionary. Apple almost always uses strings for dictionary config parameters since wrapping enums in NSNumbers is a little tedious.