Implement AWS v4 signature API

Summary:
Ref T5155. AWS introduced a hugely more-complicated "v4" API at some point, which all (I hope??) services now use.

I implemented it by following this document:

http://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html

Test Plan:

• Implemented all the examples in the document above as unit tests.
• Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5155

Differential Revision: https://secure.phabricator.com/D14978