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
adds basic custom AUTH_USER_MODEL support
makes testsuite use a custom user model if possible
don't link to user admin in menu with AUTH_USER_MODEL
Merging auth_user_model changes
Fixing username changed value
Fixing user changelist
Updating for custom auth_user_model
Merge remote-tracking branch 'upstream/develop' into develop
Updating for auth_user_model
Updating for auth_user_model
Fixing visited link colour
Fixing plugin redirects
Updating migrations for 1.5
Updating css
Updating
Tweaking cms.compat
Fixing migrations
Adding LazyProxy for PageUser