Reap workers in SIGCHLD handler

This commit tries to minimize the chance of sending a kill signal
to a process that is not a gunicorn worker by reaping children
as soon as the SIGCHLD is received.

Close #371
switch QUIT and TERM signal

With this change, when gunicorn receives a QUIT all the workers are
killed immediately and exit and TERM is used for the graceful shutdown.

Note: the old behaviour was based on the NGINX but the new one is more
correct according the following doc:

https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html

also it is complying with the way the signals are sent by heroku:

https://devcenter.heroku.com/articles/python-faq#what-constraints-exist-when-developing-applications-on-heroku
update doc for django and paster integration

gunicorn_django and gunicorn_paster commands are now deprecated so
remove them from the doc. While I am here, update the integration doc.
bad variable reference in paste startup
make sure we return the correct error with the last traceback.

fix #630
There should only be one.

gunicron_paster, gunicorn_django and other hacks are deprecated. This
patch only advertise gunicorn as the standard, stable and well supported
command.
fix title levels
Support running specific paster apps from .ini

Close #400
Remove references to deprecated console scripts
fix doc and make sure we get the --paste option
bump to R19