Support for SSE-C

Support for the recently announced Server Side Encryption with a Customer provided key (SEE-C) feature now available on Amazon S3. See the [Amazon SSE-C documentation](http://docs.aws.amazon.com/AmazonS3/latest/dev/ServerSideEncryptionCustomerKeys.html).

See [this wiki page](https://github.com/jheller/s3cmd/wiki) for usage, new options and caveats.
Calculate md5 if its still in the preserve_attrs_list, even if its not in sync_checks. This is for SSE-C which cannot use md5 for sync checks, but is still useful as in the attrs list to verify the file when its downloaded again.