Release 0.9.0

Fixed

• Check isList when determining whether to create a list-length or string-length rule.
• typing: Allow message and error functions to return null or undefined.
• Avoid incorrect TS error because $namespace option is an object that does not already contain the model's types.

Changed

• Add TEntity generic type arguments to various types
• Rename ValueType -> ValueConstructor
• Removed EntityType type alias (use EntityConstructor or EntityConstructorForType instead)

Added

• Add generic types: EntityOfType, ObjectMetaOfType, TypeOfType, etc.
• Add Model.create method with better type checking and inferencing.