Site:       Store namespace information in self._namespaces, a dict where key is a namespace ID and value is a list of
            matching names and aliases; added _get_namespaces_from_api(), namespaces(), namespace_id_to_name() and
            namespace_name_to_id(); get_page() and get_category() are smarter;
Constants:  new module, with 18 variables starting with "NS_" that hold IDs of common namespaces, e.g NS_USER = 2,
            NS_PROJECT = 4;
Exceptions: added NamespaceNotFoundError, raised by Site when bad input is given to namespace_id_to_name() or
            namespace_name_to_id();
User:       self.name -> self._name; new name() method returns name from API; dropping "get" from methods that return
            just a variable;
Category:   get_members() -> members().

User: added .get_rights() (working) and .exists() (skeleton).
Page: added .exists() (skeleton); store text as ._content; get() has a force_reload argument.
Category: fixed missing self in .get_members().
Site: self.__api -> self._api; self.__sql -> self._sql

tools.get_site() returns an actual Site object, thanks to tools.functions._get_site_object_from_dict().
Site objects now have a working (but primitive) .api_query(), .get_page(), .get_category(), and .get_user().
Page objects now have a working .get(), for getting page content from the API.
Category is now a subclass of Page, and has its own .get_members(), which returns a list of titles.

Still need to implement proper namespace logic in pages.