Tutorial improvements

[imcotton]

Several AJAX related improvements:

• Use promise callbacks from jqXHR
• Use jQuery.proxy instead of Function.prototype.bind
• Use shortcuts of jQuery.ajax
• Remove redundant state updates within post succeed
• Fetch comments only after each call has finished

Feel free to drop any suggestions, I'd happy to making rebase in order to trim it final.

[zpao]

It's been a few months and I think I'm just going to say meh to this. Parts are a bit simpler but otherwise I don't think it adds much. It might make sense to pull a couple of these ideas into a new version of the tutorial though. We can revisit if/when we do that. Thanks though!