Populate S3 Bucket Location Constraint With Region

Adds a plugin to the S3 client. When running the Aws::S3#create_bucket
operation, it will populate the location_constraint parameter with the
region if you are not creating a bucket in the default region and have
not specified a location constraint already.

Resolves GitHub Issue #47
Use Struct Syntax for Context Config

In the s3_location_constraint plugin, use struct syntax over hash syntax
for access to the context.config object.
Review Feedback for Location Constraint Plugin

- Uses optional assignment for dealing with a possible null hash.
- Fixes handler priority.
- Add a new S3 fixture for an implied location constraint.
Merge pull request #48 from aws/issue47

Populate S3 Bucket Location Constraint With Region

Fixes #47