Expiration required for signed URLS, CDN Domain settings not honored

This patch fixes two problems I discovered through doing some digging:

1. In order to create a proper signed URL, the "expires" header is needed by the AmazonS3 API. Unfortunately, the AmazonS3StreamWrapper class does not pass this argument onto the s3.class.php function authenticate(), which is required:

// If we're generating a URL, return the URL to the calling method.
		if (isset($opt['preauth']) && (integer) $opt['preauth'] > 0)
		{
			$query_params = array(
				'AWSAccessKeyId' => $this->key,
				'Expires' => $headers['Expires'],
				'Signature' => $signature,
			);

2. When a CDN Domain Name is set in AmazonS3 settings, it is only honored if https is set to TRUE and and Cloudfront is also set to true. We're not using Cloudfront, so the two being mutually exclusive proved problematic. The way I've fixed this initially is to do a simply string replacement. This was how other modules have done it, though I believe it may be possible to pass it into the API correctly.

I'd love feedback on this patch, even if there is a better way to address #2. I have yet to add it to production but so far it seems to work well.