Add the ability to cache returned tokens in the STSTokenValidator

This task is to add the ability to cache SAML tokens that are returned from an STS instance in the STSTokenValidator. An "Associated Hash" int is added to the CXF SecurityToken class. On receiving a UsernameToken which is to be dispatched to an STS for validation via WS-Trust, if it is transformed into a SAML Assertion a hash of the UsernameToken is associated with the returned Assertion, which is stored in the TokenStore.