show comments and drafts count in changes browser

- for each file in the changes browser
- made ReviewCommentSink observable (to be notified of new or
  deleted/removed comments)

Closes: #80

Screenshot:
![screen shot 2014-06-10 at 09 46 22 am](https://cloud.githubusercontent.com/assets/3630295/3233529/15cf7c54-f0bf-11e3-8de8-ab9b369b3668.png)