Support Binary data type

As of August 2012, there's a Binary data type supported in DynamoDB, and data needs to be Base64 encoded in the JSON structure sent to Amazon, and decoded coming back. 

I've made changes to support this data type and added several tests to t/01.t to test binary data going back and forth.  I'd be happy to make any changes you suggest or squash this into a single commit if you'd prefer.