Pass two — the builders and the gates (v0.5.20, DONE)

Pass two, done: the test harness that was swallowing async failures (and the real bug it was hiding), one test file split into six suites and a runner, a self-test for validate.js, the three book builders folded into admin/build/bookkit/ with both PDFs verified page-for-page, and the two writers that were fighting over book.json.