Feature Request: add RDS snapshot describe / restore methods.

I want more RDS API methods, DescribeDBSnapshots and RestoreDBInstanceFromDBSnapshot methods.
http://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DBSnapshot.html
http://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_DescribeDBInstances.html
http://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_RestoreDBInstanceFromDBSnapshot.html
so I created this patch.

Please review and merge it. :)