Implemented Joi validation/toJSONSchema with legacy fallback. See #926.
Implemented Foxx model auto-validation with joi.
Don't clone objects just to throw them away.
Added documentation for errors and isValid.
Docs consistency.
Adjusted Foxx Model docs to use joi instead of legacy schema.
Fixed typo.
Clarified isEmpty check.
Marked old Model.extend syntax for deprecation.
Merge pull request #932 from triAGENS/foxx-joiful-models

Use Joi for model validation (fixes #926)