load-global-conf now looks for /etc/awful-picman.conf

Add load-user-conf for ~/.awful-picman.conf, add debug messages and
refactor things a little bit.
Require awful 0.39 (for literal-script/style?)
Sort pics in folders according to string<

Cameras usually name pic files by appending a numeric suffix to
filenames.  By sorting with string< we get pics displayed in the order
they were captured.