Use the django_polymorphic from PyPI.

The 0.5.1 version has been released for a while now, there is no reason
to use the development version anymore.
Specify minimum versions of dependencies.

Instead of pinning exact versions.  Several of our dependencies have
already had several updates and we shouldn't force our users to use old
versions of those packages.
Don't depend on polymorphic any more

Only django-filer needs it, and it already has it as a dependency
use unicode format strings
Merge pull request #226 from fusionbox/update_filer_dep

Only pin minimum versions of our dependencies
fancy demo design
use mezzanine for the demo homepage
FIX: widgy.py was clashing with widgy package
Remove unused import
Add the templates for the demo menu
Remove unneeded bootstrap assets
Remove duplicate manage.py
Overhauled the models for the demo project.

I deleted all of the silly example models and wrote out a realistic
HomeLayout with a Slideshow and Box widgets.  This commit also throws
away the migrations that we used to have in the demo project, which is
ok, because it's a demo project.
Use the menus that widgy_mezzanine provides.

I decided to use the menus that widgy_mezzanine provides (dropdown.html
and leftnav.html) instead of providing the demo projects own menus.  I
felt that those templates were very similar to the built-ins and were
just a distraction.
Add metadata block to demo/base.html
Added the pages/index.html for HomeLayout in demo.
Move demo_widgets templates to demo_widgets app.
Remove dependency on cachebuster for demo
demo site design
responsive CSS improvements, top and side nav fixes, favicon