Content scale changes and fixes.

This ended up being a much bigger patch than I wanted it to be since it dragged on for a couple weeks. This is related to the physics/SpriteBuilder issue here: https://github.com/cocos2d/cocos2d-iphone/issues/371

* CCDirector.positionScaleFactor was renamed to UIScaleFactor
* CCDirector.contentScaleFactor is no longer tied to "retina" screens. Instead you can set the pixels:points ratio manually.
* The Mac version now respects the content scale.
* The Mac version now uses the inverse projection to convert UI coordinates like the iOS version.
* Replaced the CCResolution type with an explicit contentScale value for each CCTexture loaded. This makes it possible to load a low res fallback texture on a high res screen if you want.
* Added the ability to set the base contentScale of textures. As an example, this is so you can tell it that -ipad textures are to be treated as having a contentScale of 2.0, and -ipadhd are 4.0.

I can try splitting it up into separate pull requests if that makes it easier to review.