Download audit reports, reusing signature and response parsing logic

[ejegg]

Merchants using Amazon Pay would benefit from an easy way to download audit reports. Much of the request preparation and response parsing logic from the payments client could be reused to enable easy report downloads. See https://github.com/ejegg/login-and-pay-with-amazon-sdk-php/tree/master for a working implementation.

If Amazon is interested in this functionality, I can rebase that work onto 3.1.0.