Merge branch `events` into `master`.
Notes on an externalized `event` constructor.
Handle error events.

Closes #89.
Closes #83.
Merge branch `error-events` into `master`.
Update change log.
Release 0.0.14.
Merge branch `release-0.0.14` into `master`.
Set arity, specify arity in event handlers.

Closes #91.
Tidy `.gitignore`.

I'm ignoring AppleTalk gibberish files globally.
Further tidy of `.gitignore`.

There is never a `.proof` directory created.
Specify zero-to-many in single constructor.

Instead of returning a builder that is then only allowed to be called
once to create a zero-to-many arrayed callback, a zero-to-many callback
is created by calling `step` with two arrays.

Closes #81.
Add test for sub-cadence error handling.

Closes #79.
Notes on callbacks after step completion.
Update change log.
Release 0.0.15.
Merge branch `release-0.0.15` into `master`.
Add NPM badge.
Corrected NPM badge.
Rewrite introduction, fix for code changes.

Rewrote the introduction. Fixed it for changes to examples. We now
require that the user uses `step` to define a main cadence.
Spell check `README.md`.

To remove the one spelling error I added in the last commit.

See #92.