Chaining composition / Inheritance should not clear the previously defined links

Hey Nick,

I'm basically reopening this because I'm not sure if you saw the [last message](https://github.com/apotonick/roar/pull/54#issuecomment-12791668) on the other request, which I think isn't as intended, because the inheritance test fails.

Sorry if this is a duplicate!

Edit: I'm keeping the old failing test (extend) because I believe they are related.