Support WS-SecurityPolicy SamlToken expressions

This task is to add support to CXF for WS-SecurityPolicy SamlToken assertions. CXF 2.4 will ship with WSS4J 1.6, which gives much better SAML support via an upgrade to OpenSaml2. Support for SAML tokens in WS-SecurityPolicy is currently limited to the IssuedToken policy assertion, and so we need to add support for the SamlToken assertion, so that for example we can define SAML Assertions as a SupportingToken.