s3 COPY command support

Hi there,

First of all, great work on this tool, i love it!

I was looking into getting the supported COPY command working so i could use s3cmd.rb to create incremental backups of my s3synced folder.
I made a small patch to support this feature and i thought it might be useful for others as well.

I made a patch based on the repos: http://code.google.com/p/s3sync-s3cmd/source/checkout
The patch can be found here: http://www.good2go.nl/s3_copy.diff

Feel free to review the code or come with any improvements.