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.
Use email.utils.formatdate in gunicorn.util.http_date.

There is a also an open issue for CPython:

http://bugs.python.org/issue747320
Merge pull request #702 from berkerpeksag/use-email-utils-formatdate

Use email.utils.formatdate in gunicorn.util.http_date.
Merge pull request #701 from wking/700-debug-preload

Deprecate the --debug setting
arbiter: Consolidate configuration logging

Benoit prefers the log prefix for the first line ("Current
configuration:") but not subsequent lines (" {config}: {value}") [1],
so consolidate to a single log.debug call.  The newer '{0}'.format()
syntax requires Python 2.6 [2], but our setup.py only claims
compatibility with 2.6, 2.7, 3.2, and 3.3, so that should be fine.

[1]: https://github.com/benoitc/gunicorn/pull/701/files#r9767234
[2]: http://docs.python.org/2/whatsnew/2.6.html#pep-3101-advanced-string-formatting
Merge pull request #703 from wking/701-consolidate-config-log

arbiter: Consolidate configuration logging