Scaling of low-res images on Retina devices

This patch attempts to solve [#1118][issue] in a reasonably clean way. The main idea is to separate the concept of screen pixels from texture data pixels. By not assuming they always have a 1:1 ratio to each other (and thus a 2:1 ratio to retina points), low-res images can be given a different ratio to keep them at a constant physical size.

I've done this with a set of macros that bypass point-pixel unit conversion on low-res textures, giving their pixels a 1:1 ratio to points.

Sprites, sprite sheets, and bitmap fonts have been modified to use the new macros and seem to be working for what I'm using them for, but I've almost certainly missed some things. I only took a cursory look at TMX maps and haven't implemented anything for them yet.

Thanks for the great framework!

[issue]: http://code.google.com/p/cocos2d-iphone/issues/detail?id=1118