Example: Adding keyboard access to backbone

As @ColinEberhardt requested in issue #1017, I altered the backbone.js application to show one approach (the one I would most likely pursue) to make an MVCTodo app accessible to keyboard-only users. **I'm submitting this PR only for illustration**: I have NOT run the Selenium tests or tested the behavior across browsers. I am also NOT squashing my commits in case you'd like to examine the changes one at a time.

I made a to-do list, based on the [Application Specification](https://github.com/tastejs/todomvc/blob/master/app-spec.md#functionality), and used it in some screencasts I took of the backbone app running locally, before and after my edits, with keystrokes shown on the screen.

[Without accessibility changes](http://screencast.com/t/CUFHQJ3zm)
[With changes](http://screencast.com/t/hbDYC42QRZTE)

Please let me know what you think of the 
If we ship it like this, we'll need to make sure to create a proper release for this on the `todomvc-common` side.