Configure::read('App.wwwRoot')

Instead of `App.www_root` this should be consistent with other keys like `App.imageBaseUrl` and `App.fullBaseUrl` and all other config keys - so camelBacked as `App.wwwRoot`.

There are also a few constant usages left in 3.0. E.g.:

    $webrootPath = WWW_ROOT . str_replace('/', DS, $filepath);

Should those also use the Configure option instead?