160 - Intermediary API

[nezaniel]

This provides an intermediary API package. Its purpose is

• property validation
• property un/serialization
• high level read model instantiation

Thus, the following major adjustments were made:

• NodeInterface now deals with SerializedPropertyValues instead of PropertyCollectionInterface
• high-level commands like CreateNodeAggregateWithNode and their respective handler methods have been moved to the Intermediary
• high-level read models like PropertyCollection, TraversableNodeInterface (now NodeBasedReadModelInterface) or TraversableNodes have been moved to the intermediary
• high-level read model instantiation has been moved to the Intermediary
• PropertyValuesToWrite and serializers have been moved to the Intermediary