Additional tests for text asset autosave

These should cover all the cases brainstormed at
https://github.com/PitonFoundation/atlas/issues/848#issuecomment-26346487

Addresses #848
Trigger autosave on keyup and initialize form less often

Trigger text asset autosave on keyup events instead of the editor's
blur event.  This is needed because the editor wouldn't always lose
focus when we needed to autosave, such as when switching sections.
So, listen to keyup events on the editor element and pass them
through to the underlying textarea, which in turn triggers the
change event on the Backbone Form view, which we bind to with the
autosave logic.

In the process of implementing this, I found that we were calling
``SectionAssetEditView.initializeForm()`` all over the place.
This was problematic because one place where the form was initialized
was on the model's ``change`` event. Since we're autosaving the model,
the form would get clobbered each time the model autosaves. In general,
it's not great to have the overhead of recreating and re-rendering
the view when it's not needed.

As far as I can tell, it only needs to be called when we're ready
to edit the asset. So, I removed all the calls and added one to
``EditState.enter()``.

Addresses #848
Cleanup autosave event  binding

* s/preventDelayedSave/cancelAutosave/
* Factor event handler for ``body:change`` form event into
  ``SectionAssetEditView.autosave()``

Addresses #848
Clean up text asset autosave tests

Eliminate some of the boilerplate, bootstrapping tasks.
Make test text distinct for each test to make it easier to keep
track of where one is when stepping through the tests and to make
it easier to run tests in succession.

Addresses #848
Initialize asset editing form just before rendering

This preserves the state of the rich text editor, even when the
view is re-rendered.
Fix typo in test scenario

Addresses #848
Update changelog, upgrading instructions

Addresses #660, #844, #847, #848
Test scenario for long section scrolling in viewer

Addresses #852
Fix typo in text asset autosave test script

Addresses #848
Scroll to top of window when switching viewer sections

Update ``LinearViewerApp.showActiveSection()`` so it makes a
a call to ``window.scrollTo()`` to make sure that the user
always starts viewing a section at the top.

Add an acceptance test that explicitly checks viewing the story
in the popup viewer.

Addresses #852
Take care around JavaScript reserved word "class"

Addresses #884
Merge branch 'master' into develop

Merge in fix for #884

Conflicts:
	README.rst
Merge branch 'develop' into 852-viewer_scrolling
Viewer JavaScript indentation and jshint cleanup
Remove outdated failing JavaScript tests

As of 55408bd5c5f52ea963d6d8776f1a1a39d402f0b5, the form for
a ``SectionAssetEditView`` instance is initialized right before
rendering, so its no longer correct to test for it on initialization.

The behavior of widget.js is that empty storybase-story-list-embed
divs won't be processed as they don't have any URL providing elements
in them. This seems like fine behavior,
so remove the "replaces empty divs with a class of
'storybase-story-list-embed' with iframe ..." spec

jshint cleanup of spec files, especially lots of forgetting to include
'()'s in calls like ``new ClassName()`` and trailing semicolons.
Merge branch 'develop' into 852-viewer_scrolling