Implement site-wide configuration settings

* add site_config_dirs() to appdirs to determine locations across OSes
* add system_config_files to locations.py
* add system_config_files to get_config_files() and re-order files entries to correct precedence
* document changes to configuration files in user guide

Closes #309
Nit pick, couldn't this just be moved up top so that it says ``xdg_config_dirs = os.getenv('XDG_CONFIG_DIRS', '/etc/xdg')``? I think that follows the spec better since it states that ``/etc/xdg`` is the *default* if ``XDG_CONFIG_DIRS`` is not set.