xAPI validator: Clean up actor > id validation.
xAPI validator: Fix wrong validation for statement > actor > ID -- Actor can be a group.
xAPI validator: Validate actor.member.
xAPI validator: Test statement > actor > mbox format.
xAPI validator: Try to make Jenkins be happy again.
xAPI validator: Clean up validateParts() method.
xAPI validator: Validate statement > attachment > contentType.
xAPI validator: Validate statement > attachment > length.
xAPI validator: Validate statement > attachment > sha2 is required.
xAPI validator: Validate statement > attachment > usageType is required.
xAPI validator: Validate statement > attachment > sha2 must be a valid base64 string.