3.0 class name

Refs https://groups.google.com/forum/#!topic/cake-php/STfLJ6vXcHE

Unifies way "className" is used and spelled across the framework.
Especially since it is already camelBacked in `public function getRealClassName($type, $class) {}` etc.

It was actually already called as `App::className()` in several places.

One thing yet missing is `Configure::write('Acl.classname', 'DbAcl');` - should we do that afterwards in a separate PR maybe?