Removing `#import`s from the pch and import finer scopes

Decided in the spirit of getting imports right (and making things leaner overall) to remove the UIKit/Foundation/category additions from the Prefix.pch file and instead get my per-file #imports right (heck, as it was I didn't need any imports in about half my headers).

As a result… a surprising few things broke.  What did boiled down to these:
• Missing UIColor+SDTAdditions, UIViewController+SDTAdditions, and CGAdditions addition header imports
• Missing <CoreGraphics/CGBase.h>, …/CGGeometry.h>, and …/CGImage.h> for CGFloat, GCRect, CGSize, CGPoint, CGImageAlphaInfo, CGBitmapInfo.
• More-specific and missing <UIKit/UIColor.h>, <UIKit/UIView.h>, 

For <Foundation/…>, I've mostly decided to include it all (`<Foundation/Foundation.h>`) since most of the time we need one thing from it, we need a lot.  In a couple of instances, I've only included NSString, but that's not the norm.

Also, for `.m` files I've opted to include a whole framework (`<Foundation/Foundation.h>` or `<UIKit/UIKit.h>`) more liberally since the weight on the build is much less significant and it can become a pain to have to add an import for every new class one uses.

In a few spots, I've eliminated forward declarations for exact imports.  This seems to be the popular (and less headache-inducing) approach for Obj-C.  Well, maybe not the exact part, but that's me.