* Site: New _get_logged_in_user() method, name self-explanatory. This acts
  as a replacement for the former crud in get_user(), which now calls this
  when the username arg is None. This method will first try to determine
  our username based on a special cookie in self._cookiejar (cookie.name is
  self._name + "UserName", e.g. "enwikiUserName"), and will only do an API
  query if no cookie was found. This removes an API query that is usually
  only necessary if we are not logged in.
* Site: silly bugfix in __init__().
* User: Reverted earlier change to _get_attribute() (addition of
  raise_exception arg); name() and exists() now use their own code, which
  is simpler.
* User: Calling name() does not do an API query unless force=True, unlike
  the other "get" methods.
* User: .join() instead of .format() because I feel it looks cleaner and is
  probably more efficient.

* Site's __init__() takes more args, all optional. As long as enough are
  provided to do an API query, the missing ones will be filled in
  automatically by _load_attributes(), which is called in __init__().
* User: _get_attribute_from_api() -> _get_attribute();
  _load_attributes_from_api() -> _load_attributes.
* Sites in config.json are stored with different keys/values.

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().

* Got rid of ConfigError from exceptions.py.
* Try to load config ourselves if it isn't already, via the new _load_config()
  method of Site. It uses getpass if passwords are encrypted, as done by
  earwigbot.py.
* Cleaned up UserNotFoundError in user.py and exceptions.py.

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