hook: registering hooks is made via Pipeline.
test: referenced via respective index.js.

This is convenient to quickly enable / disable whole chunks of tests.
open implementation, nearly done!

It does not normalize correctly pixel data for colo mapped images (8-bit PNG or GIF).
I've posted a issue to Leptonica about this: https://code.google.com/p/leptonica/issues/detail?id=82.