allow for push notifications in requestAndDownloadReport

[ericblade]

Per #111 @pwrnrd mentioned

Btw, if your looking at reporting, what might be very nice to implement is this: https://docs.developer.amazonservices.com/en_US/notifications/Notifications_ReportProcessingFinishedNotification.html .

it looks like we could implement a SQS queue destination as a parameter, then we can watch that instead of polling.

perhaps making a separate package that implements more complex things like that is better than having it inside the main package, especially since going to SQS would add a dependency to the AWS libraries.