Linux fixes

Fixes Issue #16
Bumps version to 1.3.0
Removes gratuitous case mangling on the case-sensitive Linux, since that's not wise
Fixes the uterly wrong behaviour in site_data_dir, return result based on XDG_DATA_DIRS and make room for respecting the standard which specifies XDG_DATA_DIRS is a multiple-value variable
Add *_config_dir which are distinct on nix-es, according to XDG specs; on Windows and Mac return the corresponding *_data_dir (Fixes Issue #6)