Add JSON schema support for validation and relationships decoration

[mnugter]

1. Define a superset for JSON schema which adds cardinality support in #ref tags.
2. Create a factory method to create the relationship decorator from a JSON schema
3. Add Resource decorator for validating data when setting the data property.

Create this as a separate package. The package should introduce a helper method that accepts a JSON schema and initializes the relationship and validator decorators on the Resource.

[mnugter]

As a JSON schema is a description of a single resource adding support via the repository seems logical.

Relationships:

• Somehow add the ability to register a JSON schema on repository level (use a decorator that adds this feature)
• Then add a Resource decorator to read the relationships from JSON schema you can get via the repository manager

Validation:

• Find a good validation library and add a decorator that validates the data on setting the data property.