Update events in `README.md`.

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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.

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index.js                 11004/10.75  3583/03.50  3120/03.05  1327/01.30
Merge branch `events` into `master`.