Add getBucket method to S3 adapter

Sometimes you want to get the bucket name from your S3 adapter. For example in this situation (see @jeremeamia's reply): http://stackoverflow.com/questions/25323753/laravel-league-flysystem-getting-file-url-with-aws-s3

It would be even cooler if the adapter supported the getObjectUrl call easier. Can I send in a PR for that?