Aws-mfa-v2

Fixed incorrect dependencies.

Fixes incorrectly listed dependencies issue.

The core script has been largely rewritten. Also tests have been added and code coverage is over 90%.

Minor bug fixes and logic cleanup.

Basic functionality is complete. No tests and not published to pypi yet.

Features:
* Obtain STS credentials and store them in ~/.aws/credentials
* YubiKey OATH token generation support