Adding credentials file support

AWS recently announced unified credentials files for all their supported SDKs (http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs). 

I added support for this in goamz, since I personally need this support for a project I'm working on.