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.
This changes the checking of SSE-C file uploads.  We can't compare the returned ETag to the local file md5 hash for SSE-C objects. Instead, verify that the returned md5 hash of the SSE-C key is the same as the one we sent.

Not perfect, but that is all the information that is available to verify.