Use ES6 promises

[danvk]

I suspect Flow will understand them better than it understands Q.

[danvk]

Cheat sheet:

• Q.when(x) → Promise.resolve(x)
• Q.reject(x) → Promise.reject(x)
• Q.all([p1, p2, ...]) → Promise.all([p1, p2, ....])

For Q.defer(), there's promise-defer.

Conveying progress (.notify()), there is no ES6 equivalent. Unfortunately, this may be a deal-breaker. We use promise notifications to indicate how many BAM requests have been made, for example.