Several bug fixes, documentation fixes, and additions.

(fix) critical error when `additionalProperties: true` 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`.