Add banner when remote changes occur

Something like "This content has been changed. Please refresh the page"

TODO:

- [x] test that the polling stops when the current page or book changes
- [ ] test with a version of `cnx-authoring` that supports ETags
- [x] add the banner div instead of an alert

In order to work without code changes to `cnx-authoring` you will need to add `If-None-Match` to `cors.access_control_allow_headers` in `development.ini`.

Screenshot:

![image](https://cloud.githubusercontent.com/assets/253202/2848712/d5db30f4-d0c3-11e3-8965-fbea3dd79c95.png)