Improved debug logging

• SDTLog prepends with file name, line number, class name, instance address, and method name
• DebugBlock (from http://stackoverflow.com/questions/8783363/ios-debugblock-macro) provides an Obj-C block that gets compiled out in non-debug builds
Cleaned up drawing code in ArenaVisRep and SceneView

• Split guessScreenPPI into screenPPI and screenPPIRetinaDisregarded, both self-memoized (rather than inside of init) and the former using the result of the later.
• GCContext scaling and translating is now done by SceneView#drawContentLayerWithin: instead of ArenaVisRep#draw (formerly drawAtPPI:forViewFocus:inContext).
	• I'm not sure if this is in opposition to an earlier design intent, but it works better, and I'll likely be splitting things out to additional classes soon anyway.
• SceneView's drawing is a little more broken down now. The UIKit-ordained drawRect: now calls three “layer” draws (drawBackLayerWithin:, drawContentLayerWithin:, and drawOverlayLayerWithin:), which are each intended to be drawn with different dimensional, effect, and optimization intents. Those then call out to (or for now, contain) drawing code.
• Cleaned up blank-line spacing in ArenaVisRep and SceneView, giving it my normal amount of breathing room (2 between most methods), and a little extra here and there for readability.
Changed to more definite underscore-delimited post-hungarian notation.

• I think I messed up something in my last commit because I forgot that the ‘_in’ suffix (like in ‘kCenterIn’) meant inches, and not just inward or inside. I was thinking about changing it to a verbose ‘Inches’, but this is my project and I love micro-typing, so I'm going to try out this style (‘_in’ suffixed) instead.
• Same for ‘Px’, now ‘_px’.
• If I could use ‘+’ in identifiers, I totally would. It'd be like e-mail!
• ‘center.k.in’ would be pretty cool too, like a file extension.
Got rid of the pulsating background and difference blending

They were slow and kind of annoying. Reduced the backgroundColor method to an init line as well.

Also, removed the preferredFramesPerSecond setting of 30 in AppDelegate, defaulting back to 60. I'm not sure why I did that.
Fixed overlay circle's edge color.

Accidentally was calling ‘setFill’, not ‘setStroke’.