Added support to set custom headers to AWS-S3

@davsclaus 

This is especially needed to send encryption headers to S3 among others (http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingKMSEncryption.html). I did try to upgrade the AWS SDK to 1.9.3 which has Header constants, but that breaks the DynamoDB which would need to be upgraded to v2. I will work on it as I get time, but for now this is a solution.

Since, this is my first pull request, please let me know if I need to do/submit anything else. Thanks.