AmazonNNNClientFactory: Allow default credentials

* Allow using SQS and SNS connections using default credentials.
* This is the case when, for example, starting an ec2 instance using an IAM-role, where these credentaisl are implicit from the instance.
* This way, we would not need to have the accesskey and secretkey in config files (yay!)
* This commit also deprecates the caching factories in AmazonSQSClientFactory and AmazonSNSClientFactory (we should handle that in the caller instead).
Merge pull request #27 from iZettle/sqs-sns-client-factories-allow-default-credentials

AmazonNNNClientFactory: Allow default credentials