Add a teaser model

Teaser is a per-language, short description of a page.  I had to
override the ``PageAdmin`` class for Django CMS pages to include the
teaser in the Page admin.

Addresses #426
Added __unicode__ method, docstrings to Teaser

Add a __unicode__ method and some docstrings and comments to the Teaser
model and the custom admin.

Addresses #426
Update changelog and docs for teaser feature
Force section list to full width

Set width of the ``#section-list`` container to take up the entire available
horizontal space.  This was the behavior but in Chrome 0.25, the section
list started shrinking to show only a single slide icon.

Addresses #648
Supress error for many assets, same container

Catch the MultipleObjectsReturned execption, log an error, and pick one
of the multiple assets so the viewer sees something.

I had to add the section model instance to the context when calling the
``container`` template tag so I could include the section ID in the error
log and make it easier to look up and fix stories when this occurs.

Addresses #534
Keep track of seen, uncaught JavaScript errors

Keep track of seen, uncaught JavaScript errors and only post them to the
server if an error has never been seen before.

This should reduce the amount of traffic to the server, the amount of
email if we're doing email notifications on these kinds of errors, and the
load on the browser making all those requests.

Closes #635
Allow superusers to open any story in builder

By default, the list endpoints for stories, assets and datasets filter the
lists to only published items for unauthenticated users and to only a
user's items for authenticated users.  This prevents superusers from
opening another user's story in the builder, making it harder to debug
problems.  This commit allows superusers to see an unfiltered list of items
via API endpoints by removing all queryset filters in the various
``Resource`` subclassses.

Additional testing is needed before *editing* stories by other users
(including superusers) is supported, but this should at least allow us to
investigate JavaScript errors thrown by the builder.

Addresses #657