Allow binary key values via RESTful API

[tmaiaroto]

The CLI lets one provide a data file to use for the key value. Since everything is stored as binary in DynamoDB (and ultimately S3 and hopefully anything else that ends up being a storage engine) it's easy to do this.

However, API Gateway doesn't allow binary data to be sent through it. So that's a bummer - there wouldn't be parity between the local API server, the CLI, and then AWS API Gateway.

AWS is hopefully changing this in the future. I'm not sure it's worth exploring until they do. This isn't a huge issue or major use case.

[tmaiaroto]

Note: This will work fine through API Gateway too. It wouldn't before, but it will now that API GW has binary support.