Add hi-res front and back covers

Cropped from thisnewoceanhist00swen_orig_jp2.tar
Clean up front and back cover somewhat
Crop the front and back cover to 2:3 aspect ratio
Add back cover to EPUB
Add missing type attribute
Add new crop of pig drop sketch, in color

http://archive.org/stream/thisnewoceanhist00swen#page/144/mode/1up