Clean up introduction
Better indexing for Chapter 1
Add index entries for Chapter 2
Add index terms for Chapter 3
Fix slightly tilted arrow in illustration
Suppress autogenerated section IDs
Add a script to extract the solutions, and put them at the end of the book

Also fix up some problems with hyperrefs

Closes #85
Escape commas in index references to names
Move exercises.html to code/index.html, also add project file downloads
Add index terms to Chapter 4
Fix check_links script to deal with new doc ids