VerifySignature respects namespaces (and some refactoring)

1. Respect namespaces in document (whoa, they can be named differently!)
2. Allow to change used namespaces (if anyone will ever need)
3. Some refactoring, cleanup, and comments

Sorry, no tests yet (I've only idea to test it — to provide valid and invalid sample XMLs).

What do you think in common?

When I've tried to use `Akami::WSSE::VerifySignature` today, it wasn't working at all. I've fixed it and also did some refactoring (сould not resist the temptation). Now it works for messages I send and receive.