Add graceful_signals configuration value.

Gunicorn performs a clean shutdown on SIGQUIT, and a hard shutdown on
SIGTERM.  However, Heroku uses SIGTERM to request a clean shutdown, not
SIGQUIT.  (Hard shutdown is SIGKILL.)  This allows specifying which
signals should perform a clean shutdown:

import signal
graceful_signals = [signal.SIGTERM]

The default value is [signal.SIGQUIT], which is the existing behavior.