Replace class to_json method by instance method
Create correct sub classes of Manifest
Replace check for schema_type by Polymorphism
Get rid of Manifest.schema_type
Compute validity of results from error array
Move schema ids to sub classes
Iterate over subclasses to create Manifest objects
Define Manifest attributes programmatically

Add JsonObject. This class dynamically creates a hierarchy of sub
classes from a DSL definition, which reads and writes corresponding
JSON data. With the "attribute" helper the attributes of the class
are defined.

Move Manifest to JsonObject. This simplifies the class significantly
as the attribute only has to defined once with the "attribute"
helper and no other code is necessary to read or write JSON.

Simplify code dealing with Manifests. Some of the code can be
simplified now as the attributes are now accessible as meta data.