Remove automatic example validation

[hueniverse]

Consider: Joi.number().min(1).example(3).min(10). Examples are set as flags, not rules. This means they apply to the schema as a whole. It should not pass validation simply because of its location in the chain. It should also not just stay as-is of the schema is then used as the base of something more complex.

There isn't a simple solution. We should not revalidate the examples on every new rule.

I also dislike the solution for context and references, especially now that you can provide references above parents. This is all just unworkable. How do you express the "me" entry point in a grandparent reference to validate an example?

The use case for validating examples is clear - make sure the examples stay correct as the schema changes in the future (as well as to start with). Not sure yet what's the right replacement.

Breaking changes:

• no longer validated at time of schema construction
• arguments can only be plain example values, not wrapped in array
• describe() examples contain a plain array of the provided examples, not wrapped in { value }