[BC] Updating the Amazon S3 Adapter to use the new AWS SDK for PHP

This pull request updates the Amazon S3 adapter to use the new AWS SDK for PHP. This PR introduces a breaking change in that the service object passed into the AmazonS3 adapter must be a `Aws\S3\S3Client` object, and the metadata parameter options have changed to match what is available in the updated SDK. 

Note that the previous adapters that used the old SDK were moved to `LegacyAmazonS3` and `LegacyAclAwareAmazonS3`.

This closes several existing issues:

* The new SDK detects content-types by default. Closes #189
* Allows the setting of a custom Content-Type. Closes #187
* Allows a method to retrieve an object URL. Closes #149
* Use a private canned ACL by default. Closes #132