Added --flatten option for use with s3 cp and sync commands

The flatten option will flatten a directory hierarchy such that all files
copied or synced from the source directory will be copied to the root of the
destination directory specified on the command line, i.e. any sub-directories
under the source directory will not be recreated on the destination side.

This is useful for copying or syncing the files in an S3 bucket but without
recreating the directory hierarchy structure under the source dir. Obviously the
filenames should be unique to prevent overwrites.

This option can be used with data created in an S3 bucket using the ruby gem
"paperclip", in order to create a single directory containing all files.