Introduce a type factory

[Chris Müller]

With the possibility to register additional types (#38) it would be cool to create a type by name to separate the implementation from the schema type. So no code must be changed when overriding a type model implementation.

• A TypeFactory is available which takes the type name and returns the according model object.
• Throughout the code the type models are using the factory to create a model object.
• The documentation is adjusted.
• The change log is adjusted.