Added signal which is fired when server needs restart

As we all know, adding, removing or changing apphook pages requires a
server restart. Until now, the maintainer/content-editors of a site
had to do and detect this manually.

I propose we add a signal ``cms.signals.urls_need_reloading`` which
fires when the CMS detects the server needs a restart.

This signal is just sent by the django CMS. The core itself does nothing
when this signal is sent. However it allows developers to write custom
listeners to that signal that take an appropriate action in their
envirnment.

For some common use cases, there might even be generic implementations
(outside djagno-cms core) which solve this issue (eg killing os.getppid
in single-server gunicorn systems).
restored Python 2.5 compatibility
Merge branch 'develop' into ojii-urls-need-reloading-signal

Conflicts:
	cms/signals.py
	cms/tests/__init__.py
	docs/extending_cms/app_integration.rst
massive signals refactor
page moderation migration and signal refactor
signals working again
fixed tests
changelog
debug server signal
signals fixed, test fixed, testserver autoreloads apphoook... wooot
don't print restart signal for tests
fixed signal firing and reload of testserver and docs
fix py3 relaod
Merge pull request #2585 from digi604/ojii-urls-need-reloading-signal

apphook needs reloading signal