R1:2b00f5e8b66f

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`…