support InstanceProfileCredentialsProvider #14
Description
When running on an EC2 instance, you can avoid having to generate credentials using IAM instance profiles. This is safer because it allows for more precise role-based access control.
The solution would be to fall back to an InstanceProfileCredentialsProvider (example) when there are no credentials available.
See http://docs.aws.amazon.com/AWSSdkDocsJava/latest/DeveloperGuide/java-dg-roles.html, http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html for more background.