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
removed link and snippet plugin
typo in requirements
Merge branch 'develop' into link-remove
tests passing but not working yet
Merge branch 'develop' into link-remove
lazy choicefield
upated requirements
updated requirements
removed new migrations
updated docs
doc updates
intial draft
doc fix
empty choices
further changes
docs
fix tests
moved file plugin from cms to https://github.com/divio/djangocms-file