Deprecate the --debug setting

"Limits the number of worker processes to 1" hasn't been true since
06a4dc6 (fix one error in gunicorn_paster, global conf was ignored,
2010-06-22), although it was true when the line was added in 3c7d532
(Large refactor of the documentation and website, 2010-05-22).

"changes some error handling that's sent to clients" hasn't been true
since feb86d3 (don't display the traceback in the HTTP response,
2013-09-27).

The only remaining actions that --debug had were disabling --preload
and hiding debug-level config logging.  The former seems useless (just
disable --preload directly) and the latter at doesn't seem useful
enough for a new setting (just turn down --log-level).  With this
commit, --preload always works and you always get debug-level config
logging.

I left a stub Debug entry in gunicorn.config, which we can leave in
place while folks convert any gunicorn scripts and configurations to
drop --debug.  When the time comes, we can just remove that entry.  I
also the boolean-config tests to use --preload, since that will still
be around after we remove the dummy Debug entry.

Fixes #700.