Cleans up agent schema unit test
Adds test for invalid statements and makes the xAPI schema pass
Makes invalid statement in test/data/1.0.0/valid valid again.
Removes minimal-missing-id.json from test/data/1.0.0/invald/statements/ because id is recommended but not required
Removes verb from minimal-missing-verb.json
Restructures unit tests for validator
Identified group is identified by IFI not by name
Adds test for xAPI schema and Group subschema