trigger "update" event when account.update succeeds

[gr2m]

account.update(options) is implemented, but does not yet emit an update event.

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• [x] claim this issue (comment below or assign yourself) (@hipsterbrown)
• [x] add update event to README. Commit it with docs(README): update event
• [x] start a PR
• [x] update unit tests at test/unit/update-test.js to check if update event is emitted
• [x] update integration test at test/integration/events-test.js to test for update event
• [x] commit with test: update event & push
• [x] implement the update event and commit with feat: update event & push
• [x] review & merge
• [x] update https://github.com/gr2m/milestones/issues/88