Several bug fixes, documentation fixes, and additions.

* (fix) critical error when `additionalProperties: false` and a value that is not an object is given. (since Object.keys() is called)
* (fix) minItems and maxItems error messages, current length was undefined.
* (fix) dead links in validate.md
* (fix) minor documentation mismatching.
* (add) error messages for `additionalProperties` and `additionalItems`.