Issue title: Support for IAM roles
 Issue body: It would be great if you supported IAM roles for s3 authentication 

 Comments: 
 Comment 0: I will second this.  It's a simple change that is supported by Fog

 Comment 1: Ok, so i've added some fixes for this, to my forked version.  https://github.com/jforest/backup

I added the few bits I think are necessary, please let me know what you think.

I've added use_iam_profile.   

 Comment 2: Submitted a pull request, #493 

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