Change the pictures to palette, run some through optipng and/or pngout

I've optimized the .png files (almost) losslessly.

I say almost because I reduced the palettes to 256 colors (by changing mode to indexed in gimp, and leaving the defaults). As the images are mostly grayscale or uncolorful, this doesn't make much of a difference visually.

This is useful for, e.g., kindle pdf versions where (I think) size affects how quickly the pdf loads and changes pages.

Before:
120K	exercise_solutions.pdf
404K	cover-lores-back-bw.png
416K	torus-lores-bw.png
468K	cover-lores-back.png
628K	cover-lores-front.png
692K	cover-lores.png
752K	cover-lores-front-bw.png
1.3M	torus-hires-bw.png
2.1M	cover-hires-back.png
2.3M	cover-hires-back-bw.png
2.9M	cover-letter.pdf
3.6M	hott-online.pdf
3.7M	hott-ustrade.pdf
5.3M	cover-hires-front.png
5.4M	cover-hires-front-bw.png
6.0M	hott-letter.pdf
11M	cover-lulu.pdf
13M	cover-hires-bw.png
14M	cover-hires.png

After:
16K	cover-lores-back.png
16K	cover-lores-back-bw.png
24K	torus-lores-bw.png
48K	cover-lores-front-bw.png
52K	cover-lores-front.png
68K	cover-lores.png
120K	exercise_solutions.pdf
132K	torus-hires-bw.png
196K	cover-hires-back.png
196K	cover-hires-back-bw.png
696K	cover-hires-front-bw.png
732K	cover-hires-front.png
916K	cover-hires-bw.png
956K	cover-hires.png
1.6M	cover-letter.pdf
1.7M	cover-lulu.pdf
2.7M	hott-online.pdf
3.2M	hott-ustrade.pdf
4.5M	hott-letter.pdf