First version of DynamoDB Store

I modeled storehaus-dynamodb off storehaus-memcache which uses
a base client  Store[String, ChannelBuffer].

There is a base store that extends Store[String, AttributeValue]
where AttributeValue is the AWS SDK specific data type that you
use for requests to Dynamo.

DynamoStringStore and DynamoLongStore use ConversionStore
to provide a more user friendly, native type interface to
DynamoDB. DynamoLongStore is also mergeable and can be used
for incrementing atomic counters in DynamoDB.