* Site's api_query() is much smarter. It uses a custom urllib2 URL opener with cookie support and catches URLErrors, raising its own brand new exception (SiteAPIError) when something is wrong.
* The opener now uses a custom User-Agent, which is a constant in wiki.tools.constants.
* Site instances automatically login via _login(), which accepts a username and password (provided via config by get_site()) and uses two api_query()s and stores the login data as cookies in self._cookiejar. Login data is not preserved between bot restarts yet. Login errors, e.g. a bad password or username, raise the new LoginError.
* Site's get_user()'s username argument is now optional. If left blank, will return the current logged-in user, provided by an API query.
* Misc cleanup throughout.

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.