Refactor Akami::WSSE to_xml. Signature / Timestamp no longer mutually exclusive.

Ran into this when integrating with a third-party API. This pull request addresses issues raised in https://github.com/savonrb/akami/issues/15

Details:
* Clean up ugly if-statement in to_xml.
* Ensure certificate and timestamp headers are not mutually exclusive.
* Add basic signature and signature with timestamp specs.