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
Ah, I missed that note from him. I disagree though: on my Windows 7 system *every* entry in \ProgramData\ is a vendor subdir. PyPA would be our vendor subdir.