Comment tree for activities are sometimes incorrect

![screen shot 2014-10-09 at 16 38 34](https://cloud.githubusercontent.com/assets/109850/4579163/7450c5c0-4fca-11e4-9d74-9bed7a386ea3.png)

It probably has to be:
```
_findComment(comments, comments[0].inReplyTo);
```
as we need to return `inReplyTo` in case it wasn't in the `comments` set