More flexibility in custom Page types and managers

Changes:
* reformatted some code in pep8 format (120 characters)
* ability to override the PageManager from your own project
* ability to override the ContentItemManager from your own project
* ability to create subclasses of Page
* added content_type to Page model to be able to instantiate the subclass when opening a Page
* ability to change the content_type in admin to change the type of the Page
* added Page.is_public field in South migration
* added Page.content_type field in South migration
* made some performance optimization when looking for the right Page object by a singleton
* added a load_class function to be able to import classes by fully qualified classname strings