Add emoji-cdn.js to support emojis in nits

Added emoji support in nit comments by using standard emoji syntax `:turtle:`.

### Implementation details

* Emoji is only rendered in nit comments.
* I'm using https://github.com/austenito/emojify-cdn which is a fork of http://hassankhan.github.io/emojify.js/
* The following emojis are supported: http://www.emoji-cheat-sheet.com/
* The emojis are hosted here: http://www.tortue.me/emoji/[emoji_name].png and are served via Cloudflare's CDNs.

You can verify this locally by inspecting images and viewing the response headers:

![emoji_cloudflare](https://f.cloud.github.com/assets/24883/1897789/6b99e96e-7bfb-11e3-8c86-57f77028a97c.png)

So assuming you trust the emojis I'm serving, we can have emojis!!! :turtle: :flags: