Authn request extensions

This pull request adds the ability to add custom extensions to the AuthnRequest.  I chose to go with adding a setting which takes a lambda to build the extensions since the `samlp:Extensions` attribute allows for a sequence of any element type.  

The `samlp:Extensions` element comes from the `RequestAbstractType`.  More info at http://schemas.stylusstudio.com/saml/nd253446a/complexType_RequestAbstractType.html.