TMI: Documentation: what a concept

[tim]

With rustpkg I'm trying as much as I can to stick to a tests-first/docs-first approach (I suppose strictly speaking, tests-first/docs-second), so this is my pull request for today: adding a draft of a manual (without much actual content, but starting to write things down) and rearranging the test scenarios again to have the directory structure that we agreed on.

I figure when I don't know exactly what I'm doing, writing things down as I go along (rather than waiting until the end to write the docs) helps me see if there's an area I'm still not clear on. I don't know why it's so hard for me to remember that that's the case for me :-)