Add DynamoDB implementation of the ZF2 session save handler interface

The AWS PHP SDK already has a great session save handler that uses a DynamoDB backend.

All this does is implement the ZF2 session save handler interface so that it can be used like any other save handler within ZF2.

If this would be a desired addition, I think creating a factory would be a good next step for ease of integration, but I'm not sure where the session handler configuration ought to live.

Thanks for your consideration!