Ditch the `t` Suite/Spec helper.

This closes #3, modifying the API to only need to call top-level
`describe` and `it` functions. There are obviously still a lot of
improvements to make, but this gets us on the same point we were before,
only with a much nicer API.
Update the README to reflect the API changes.

I'll add examples once this project is mature enough, but it's always
nice to keep basic things up-to-date.