Use IAM-provided credentials if indicated.

When running on an EC2 instance that has [IAM profile (aka IAM role)][1], `zonify` will attempt to use IAM-provided credentials instead of expecting the usual environment variables, if `--use-iam-profile` command line option is specified.

`fog` has had this feature for a while (fog/fog#996), so `zonify` can benefit right away :smile: 

[1]: http://aws.typepad.com/aws/2012/06/iam-roles-for-ec2-instances-simplified-secure-access-to-aws-service-apis-from-ec2.html