Additional AWS::S3 options for Backend::S3.

Added the option to provide additional arguments for AWS::S3.
For example https://github.com/jubos/fake-s3 can be used now with the following setup:
```
{ secret_access_key: 'your_secret_access_key',
  access_key_id: 'your_secret_access_key',
  bucket: 'your_bucket',
  s3_port: 4567,
  s3_endpoint: 'localhost',
  use_ssl: false,
  s3_force_path_style: true }
```