Fix Travis CI Node.js 10 build.

Seems to not be able to figure out what to do if `0.10` is listed first.
Fix global variable namespace leak.

Now that Proof performs global variable namespace leak detection, I've
detected this leak.
Add `/coverage` directory to `.gitignore`.
Tidy `.gitignore`.
Move diary to `diary.markdown`.
Notes on possible `event` function.
Petty detail in example code.
Event builder no longer specific to step scope.

Closes #98.
Update events in `README.md`.

File                             Raw         Min         Zip     Min/Zip
index.js                 12218/11.93  4044/03.95  3337/03.26  1484/01.45
Implement `step.event` and `step.error`.

Implement an event handler builder function that wraps the existing
callback builder as well as a short cut function for defining error
event handlers.

Closes #99.

File                             Raw         Min         Zip     Min/Zip
index.js                 11004/10.75  3583/03.50  3120/03.05  1327/01.30
Merge branch `events` into `master`.
Reset coverage by deleting coverage directory.

Closes #95.
Update change log.
Release 0.0.16.
Merge branch `release-0.0.16` into `master`.
Notes on zero to many.
Spell check diary.
Spell check comments.
Rename `diary.md`.
Notes on initial invocation.