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.
Disable md5 checks if using SSE-C because the returned ETag is no longer an md5 hash of the object.

Instead, add the new mtime sync check.