Add support for S3 endpoint regions.

Summary:
Allows to use file storage in different Amazon S3 regions as well as it
should support different file storage services with S3 compliant API
(eg.: Bashos' Riak CS).

Test Plan:

1. Create S3 bucket in non-default AWS Region (e.g. EU/Ireland).
2. Set appropriate bucket policy to allow upload & download objects for the specified access credentials.
3. Set amazon-s3.endpoint in your configuration file to an appropriate value (e.g. s3-eu-west-1.amazonaws.com for EU region).

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3965