Notebook Updates

Safari execution error is still being worked on, but some valuable fixes have popped up through this branch.

* No more `undefined` render in application view
* Stack traces should be improved in Safari and Chrome (shows a message now in Safari)
* Sandboxes only initialise on render, which removes the creation of two iframes every time the app is started
* Use the github check an authorisation endpoint, which according to github is [for checking OAuth token validity without running afoul of normal rate limits for failed login attempts](http://developer.github.com/v3/oauth/#check-an-authorization).