Support for versioned optimistic locking a la DynamoDBMapper

[brandond]

Use a version attribute like DynamoDBMapper does to ensure that the record being updated hasn't changed server-side since last accessed by the client:

http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/DynamoDBMapper.OptimisticLocking.html

[sdavids13]

Should this be closed? I believe this capability was merged in via: #664

[ikonst]

Thanks, this was indeed merged :)