Embed multistage docs directly within the README

Accurate docs for this is now coupled to the release, and should not
live in the wiki, since claims about app_env in older versions may or
may not be accurate, but they certainly aren't at this point in the
code base.
ensure RAILS_ENV is set correctly
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)