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
I think in #1733 we decided to omit the ``"PyPA"``, it only has an affect on Windows and @pfmoore was fairly against using it. I believe he said that it's *technically* the standard, but very few people actually use it that way.