Get rid of References interface

[JeroenDeDauw]

This interface adds no value and invites issues, such as $this->references->getValueHash() in Statement, which calls getValueHash, while that method is not in the interface.

[Benestar]

Should the methods dealing with a Reference's hash stay in the ReferenceList class? Or should we move them to an external class which does indexing by hashes?

[Benestar]

Fixed in #232