Add an Amazon S3 storage engine for Phabricator

Summary: Implements an S3 storage engine option for Phabricator.
Test Plan:

• Uploaded files to S3.
• Looked at them.
• Verified they appeared in S3 using the S3 file browser.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 752