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
initial import and merge
publisher tests working... hurray
Refactor show_editable_model. Add show_editable_model_icon that adds an icon attached to model editing.
CSS edit for show_editable_model_icon
Specified file name of toolbar extension.
Merge pull request #2472 from timgraham/extension_location

Specified file name of toolbar extension.
Fix js Style
Fix js Style
Merge pull request #2473 from yakky/feature/fix_js_style

Fix js Style
Refactor and rename show_editable_model. Add other render_model* templatetags.
Update tests. Add tests for new render_model templatetags
Coverage improvement
Fix url
Cleanup
Add getter & setter for translatable_content in CMSPlugin
Typo
Added tests for get/set_translatable_content
cosmetic changes for @piquadrat