Add integration test for AM::Serialization support

...which shows that we need to always `delegate :attributes`
More integration specs
Revamp specs

* Use the new `expect().to` syntax
* Avoid general fixtures by minimizing usage of `let` and `subject`
* Define as few support classes as possible, and use them for name
  lookup only - all behavior is introduced by stubbing
Minimal requires
Merge pull request #427 from haines/specs

Revamp specs