Add RDS/DescribeEventCategories.
Add a test case for RDS/DescribeEventCategories.
Add RDS/DescribeEventSubscriptions.
Add a test case for RDS/DescribeEventSubscriptions.
Add RDS/CreateEventSubscription.
Add RDS/DeleteEventSubscription.
Add a test case for RDS/{create,delete}EventSubscription.
Add RDS/ModifyEventSubscription
Add a test case for RDS/ModifyEventSubscription.