Add SAML support for AWS Security Token Service

Includes a new AWS Security Token Service (STS) API action called
AssumeRoleWithSAML that returns a set of temporary security credentials for
users who have been authenticated via a SAML authentication response.
API Updates to AWS Identity and Access Management

With this release of AWS Identity and Access Management (IAM), you can enable
identity federation using SAML 2.0 (Security Assertion Markup Language 2.0), an
open standard. This feature enables federated single sign-on, or SSO, which lets
users log into the AWS Management Console or make programmatic calls to AWS APIs
using authentication responses (assertions) from a SAML-compliant identity
provider.