Verify signature respects namespaces (and some refactoring)

1. Respect namespaces in document
2. Allow to change used namespaces (if anyone will ever need)
3. Some refactoring, cleanup, and comments
Testing Akami::WSSE::VerifySignature with presigned messages

These XMLs are signed with a signer gem with self-signed certificate.
In future it probably should be tested with on the fly signed messages by Akami itself.
Merge pull request #11 from Envek/verify_signature

VerifySignature respects namespaces