improve routing of pages

- `\cms\page\PagePage::$pageID` now always contains the page id of the
requested page
- fallback to id as page indicator to work around problems of 3rd
parties which do not properly respect a link manipulation for menu items
- check for offline mode before validating access permissions to
prevent exposing of page permissions during offline mode
- documentation of class members
Merge pull request #48 from ray176/pageRoutingImprovements

improve routing of pages