In story viewer, loading a section updates Backbone's router location without creating a new entry in the browser's history. Addresses #487.
Create "iframe" mode for viewer (#487).

Allow for story viewer request with ?context=iframe. In this mode,
story navigation doesn't write to the browser history.

Also, take out previous "fix" for #487. The replace mode for the
router wasn't behaving consistently across browsers and other
attempts to make interaction with browser history sane weren't
working.
Merge pull request #679 from patternleaf/487-browser-history-in-iframe-viewer

In story viewer, loading a section updates Backbone's router location wi...
Clean up fix for #487

Make the assignment of the ``context`` key in the view context more
pythonic.

The assignment of the context in the code to create the viewer Backbone
view can just be a local variable.  We don't need to include it in the
namespace object.

Update the Changelog to reflect these changes.