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.
Add a new function to convert a Unix epoch time to a date/time. Used for the new 'mtime' sync check.