* Exceptions: New PermissionsError; reworded docstring of SiteAPIError.
* Site: __init__() accepts an optional cookiejar parameter, otherwise we
  use CookieJar(). Added five new cookie/username-related methods. Only
  login from __init__() if we are missing valid login cookies and a user/
  pass was provided. _login() and _logout() both try to save cookies via
  _save_cookiejar(). _load_attributes() automatically refreshes all
  attributes other than namespaces if at least one is missing, instead of
  only the missing ones. api_query() raises SiteAPIError if either
  self._base_url or self._script_path is missing. Removed some pointless
  methods and renamed one; added domain().
* Functions: _get_site_object_from_dict() is cleaner, adds our cookiejar
  to Site instances using _get_cookiejar() to load a LWPCookieJar() object
  from the ".cookies" file in our project root. The same cookiejar is
  returned for every site, enabling crosswiki login, via a global variable.
* User: Renamed some methods.
* .gitignore: Added .cookies file.

* 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.