support AWS_SECURITY_TOKEN if present in env

We take advantage of [AWS MFA](http://aws.amazon.com/iam/details/mfa/), and pass a [standard](http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs) `AWS_SESSION_TOKEN` env var to our tooling.

By adding this, our tooling that leans on `goamz` becomes MFA-enabled.

Thank you!