Now that there's an official Go AWS SDK from Amazon, use that instead of
goamz. goamz isn't getting much love these days.
Implement support for STS credentials, as in assumed roles and EC2
instance profiles. The configuration is extended to support a session
token, though I'm not sure why anyone would put temporary credentials in
a configuration file. More likely, no credentials will be explicitly
configured at all, and they will be discovered through the standard SDK
mechanisms described at
<https://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs>.
Resolves#342.
