Document changes for improved postmortem debugging.
Add RSpec integration.

This necessitated a pretty massive refactoring. The reason being that
until now I'd been using `#within_construct` inside
`config.around(:each)` hooks to do RSpec integration
per-project. However, RSpec `around` hooks don't grant access to the
current `Example`, which also means they don't have access to the
exception (if any) the current example has raised. As a result, I
decided to reimplement `#within_construct` in terms of two bookend
methods, `#setup_construct` and `#teardown_construct`. These can then
be reused in RSpec `before` and `after` hooks.