Aws profile auth

See http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs

I noticed that there wasn't an option for authenticating with AWS using the Shared INI method, so I added one.  
This should also work on Windows (where the credentials file is located in %USERPROFILE%), but I don't have a machine to test this on.

I'm new to Go, so tell me if I've made any horrible (or tiny) mistakes.

I also did not know of a good way to test this, since it requires looking at a file outside of the project.