Changes to support Encoding param

[dougis]

I have made a change to the s3sync.rb script to facilitate using the encoding param when uploading to S3 (added so that I can auto sync gzipped content for delivery via cloudfront).
Added a passed param of --encoding="<enc>" (I am using it as --encoding="gzip")

I have attached the updated file here if anyone would like to use it (or if the mods want to merge the changes into the script for general use)