Implement Spec Objects

[cebe]

For those willing to contribute:

• The task is to fill the classes in /src/spec with information in the OpenAPI specification.
• Take a class that is not implemented yet (see README) and do the following according to the spec (See @link tag in class phpdoc):
  • add @property-read phpdoc annotations for all properties defined in the spec
  • implement attributes() method and list all properties defined in the spec, add their Type as it has been done in other classes already.
  • implement performValidation() method to validate the property values.