Add support for SQS service

- Extend SprayAWSClient to also support non-json services
- Update DynamoDBClient to be compatible with the new SprayAWSClient
- Update DynamoDBClientSpec
- Add SQSClient
- Add simple test case for some SQS operations

Note that to support non-json formats the following changes were required:
- implicitly provide ResponseHandler rather than Unmarshaller
- provide an "errorResponseHandler" rather than the exception unmarshallers
- extend request to handle request objects without content using available parameters instead