Move deserialization methods to a Helper class
Extract methods to retrieve metadata not stored in the model
Mark helper methods internal for now
Make tests pass

Quick null check for PublishedPropertyType - we can't create a dummy one from a test due to some internal Umbraco dependencies, and we can't mock it as the relevant properties aren't virtual, so just adding a quick null check for now.
Merge branch 'pvc-refactor' of github.com:imulus/Archetype into pvc-refactor
Merge pull request #108 from imulus/pvc-refactor

Extract methods from ValueConverter for reuse