Allow STS.assumeRoleWithSAML to work without access key
Add AWS.SAMLCredentials credentials object

Allows users to configure credentials for a SAML-enabled
identity provider. Usage requires a RoleArn, PrincipalArn, and
base64-encoded SAMLAssertion token from the identtiy provider:

```javascript
AWS.config.credentials = new AWS.SAMLCredentials({
  RoleArn: 'arn:aws:iam::123456789:role/Role',
  PrincipalArn: 'arn:aws:iam::123456789:role/Principal',
  SAMLAssertion: 'TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5v=='
});
```