Less xmlsec paths

Roland, I'm not sure if this is a valid approach to doing so but I tried to remove knowledge about xmlsec1 from outside of sigver.py. Can you please review and merge/let me know how it should be done?

This is a preparatory step towards making what CryptoBackend to use configurable. Please let me know any thoughts you have on that subject as well. Maybe we can talk tomorrow?

/Fredrik