R1:bd3a14f248b9

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