Added prefix_path for S3 key and cleaned up sample in readme.MD

I needed to upload files to a non-root location of my S3 bucket.  This pull request includes the ability to set a prefix_path that is optionally added to the beginning basename of any files.

I also cleaned up the example to remove the S3 key argument that was deprecated in a final release and added  buffer:false to the options of gulp.src() as this plugin won't work otherwise.