dynamodb: Rework on integration and unit tests with gocheck

This PR adds:

 - gocheck integration like s3 package
 - integration tests using DynamoDB local
 - 1 bugfix found while writing integration tests @ f2b3677

See [README in dynamodb directory](https://github.com/nabeken/goamz/blob/dynamodb-gocheck/dynamodb/README.md) for more detail.