Attachment example, clarifications, minor changes

* Added an example
* Added clarifying requirements
* Added requirement to include hash in each part's header, so receiver doesn't
have to guess which hash length to use when verifying
* Renamed sha-2 and content-Type properties to sha2 and contentType for easier reference as properties (many languages don't like - in property names)

This PR should resolve: https://github.com/adlnet/xAPI-Spec/issues/73