Fix CryptoBackendXMLSecurity.validate_signature.

The validate_signature method is called with the parameters
named, so it does not work to rename the parameters from the
names used in the superclass.
Add test case for CryptoBackendXMLSecurity with PKCS#11.
Merge pull request #72 from SUNET/pyxmlsecurity_fixes

Fix pyXMLSecurity backend, and add test case.