Add MFA support to the keychain util

This adds MFA (multi factor authentication) support to aws-keychain-util using the AWS STS (Security Token Service). 

What this lets you do is register an MFA device (virtual, like google authenticator) or a physical token with your AWS IAM user, and then restrict access to the secure parts of your amazon APIs to require two factor authentication.

This fork handles maintaining the two secret values, which are a temporary key and a security token, as well as expiring them automatically.

We've also added an "env" command that allows you to set environment variables directly without having to start a full sub-shell, making this tool suitable to run from your dotfiles.