Add support for configuring AWS S3 region.

AWS S3 buckets that are created using a non-default region does need to set `region` key on `S3Client::factory()` call.

Not configuring this way throws an `Aws\S3\Exception\PermanentRedirectException` as follows:

<pre>The bucket you are attempting to access must be addressed using the specified endpoint. Please send all future requests to this endpoint: "...".</pre>

This was detailed on https://github.com/aws/aws-sdk-php-laravel/issues/13#issuecomment-22770974.

This PR depends on https://github.com/laravel/laravel/pull/3026 to be pushed.