Add comment count to issue list

I wanted to add a comment counter in the issue list view.

Please review:

1.- I added the comment count after the issue title and it's only shown when there are comments. I thought about adding a column but not sure if I like that. 

Suggestions are welcome. This is how it's shown:

![test](https://f.cloud.github.com/assets/1119272/36305/22a84424-5304-11e2-86bc-0350d2a9f40f.png)

2.- I had to patch the _getListCount method in:

    libraries/tracker/model/tracker/list.php

to clear the group because after adding comments count it was only returning 1.