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
remove the stacks package
doc updates
intial draft
doc fix
empty choices
Remove some unused imports
reimplementing jonprindivilles jstree fix #2270
Added stopPropagation() to dblclick handler

This is to address the issue of not being able to edit models emitted by CMSPlugins. Because the event was being propagated, the double-click essentially triggers editing of both the model and of the plugin. The resulting mayhem made it impossible to do either. This fix seems to allow the editing of the model, without preventing edits to the plugin (by double-clicking on something other than the model).

Signed-off-by: Martin Koistinen <mkoistinen@gmail.com>