S3 Multipart Upload API helper for initiating a multipart upload

s3.initUpload(path, cannedAcl, headers, callback)

• 'path' - the S3 Path.
• 'cannedAcl' - the S3 Canned ACL.
• 'headers' - an object containing the HTTP headers you may want to pass to the POST request, such as the x-amz-* metadata headers.
• 'callback' - the callback that is executed when the processing finishes. It has a couple of arguments: error and result.
• If there's an error, the callback receives the error argument as Error instance.
• If the error argument is null, then the response argument contains an object indicating the initialization data for the multipart upload. It has the following keys: bucket, key, uploadId - which indicate the S3 bucket, the S3 Path, and the uploadId which is used by the Multipart Upload API to reference the in progress upload.

s3.setBucket('bar');
s3.initUpload('/foo', 'public-read', {'cache-control': 'public'}, function (err, res) {
	if (err) {
		console.error(err.message);
	} else {
		console.log(res);
		/**
		 * Outputs something like:
		 * { bucket: 'bar',
		 *   key: 'foo',
		 *   uploadId: 'VXBsb2FkIElEIGZvciA2aWWpbmcncyBteS1tb3ZpZS5tMnRzIHVwbG9hZA' }
		 */
	}
});