Support Derived Keys with the Symmetric Binding + SAML Assertions

There are a couple of problems with using Derived Keys pointing towards SAML Assertions when using the symmetric binding:

1) The SymmetricBindingHandler can't handle creating a reference to SAML Assertion if the security token does not have a (un)attached Reference to the Assertion.
2) In the holder-of-key case, using a derived key will cause the holder-of-key requirements processing to fail.

Creating a JIRA + patch for this, as it depends on a fix in WSS4J 1.6.1 which is not released yet.