simplify quoting of long shell command
add some quotes to be safe
add NEWS.md to aid with upgrades
fix incorrect docs for app_subdir
remove unnecessary use of fetch
move primary and secondary config paths into _cset section

Grouping all path calculations together improves legibility and maintainability.
rename primary_config_path and secondary_config_path for clarity
simplify display of unicorn_user
add relative path calculations in preparation for pid auto-detection

Also re-order and re-structure assignment and display of variables
for legibility.
attempt to auto-detect path of pid file (fixes #7)