Add information on how to build and use validators.

include an example of creating a re-usable validator as that is
something I think is useful to do.
Add more content to the validation chapter.
Add missing content.
Content edits.
Fix review comments + add custom rules.

Add docs for using callables as validation rule as that is sure to come
in handy for some people.
Merge pull request #967 from cakephp/3.0-validation

3.0 validation