Added wrap/clamp toggle button

• Added button with label “{wrapping/clamping}” to the Main.storyboard.
	» Wired it up to a `toggleWrapClamp:` action on ViewController.
	» ViewController `wrapClampUVsToggleButton` outlet wired to it.
• New `- toggleWrapClamp:` method flips the value of the new `_wrapUVs` ivar, calls `updateWrapClampUVsToggleButton`, and triggers redraw of the dest image.
	• `- updateWrapClampUVsToggleButton` chooses the text “Wrapping UVs” or “Clamping UVs” based on the `_wrapUVs` toggle, and sets the text on the button, sizes-it-to-fit, and re-centers it horizontally.
	• Also saving and loading this into/out of `NSUserDefaults.standardUserDefaults`, in the same fashion as before.
• Removed `GEN_DEST_IMAGE_WRAP_ST` define and usage, replacing it with the new `_wrapUVs` ivar.
• Minor: Changed clang diagnostic ignore for helper functions from `-Wunneeded-internal-declaration` to `-Wunused-function`.

Bonus:
• Updated README screenshot.  YEAAH-AH-Ah-ah-ah_ah ah …
	» Bonusbonus: This time crunched down to 49% of its original size via ImageOptim!  (_i_might_need_to_go_to_bed_)
Minor: Further downscaled destImage size

• Changed the destScale override in `- destImage` from 1 to 0.5— now half-normal and quarter-retina-scale, or 1/4 normal pixel-count, 1/16 retina pixel-count.
	» It’s a test app.  UI speed is pretty important, while algorithmic performance can be profiled.