Collapsing changes

I've been loving HackerNew since I installed it, but one thing I missed was reddit-like comment collapsing from a plugin I was previously using called [Hacker News Collapse](https://chrome.google.com/webstore/detail/hacker-news-collapse/bbkfcamiocfccgmcjngdljolljhifdph).

This pull request implements HN Collapse in HackerNew. It's quite a bit less showy than the current implementation, but I think it does a good job of making comment collapsing fast and easily accessible.

Here's what it looks like before:
![Before collapse](http://i.imgur.com/FqQDLc6.png)

And after:
![After collapse](http://i.imgur.com/gUKNkqR.png)

Let me know what you think!