Implemented periodic auto-save on the Edit Quiz page

Using the method documented here:
http://adamalbrecht.com/2013/10/30/auto-save-your-model-in-angular-js-with-watch-and-debounce/

Future improvements:

- Currently not watching for changes in quiz name - only questions trigger auto-save. What needs to be done here is to have the entire quiz model be a single property on the scope, instead of storing quiz name and questions in separate properties.

- No validation of any sort yet.

- We're not tracking changes, which may be problematic if user goes in to mess around and deletes a bunch of questions, without actually intending to save the changes. There's no way to undo anything yet.

- New quizzes are not auto-saved until the user explicitly clicks on 'Save'. We may want to auto-save new quizzes as well, but in that case, we may want to pass in a default value for quiz name.