Adapt the show parent hover to display all parents of the comment.

Main changes are:

* Adapt the show parent hover to display all parents of the comment.
* Add a keyboard shortcut to trigger the hover. (default is `shift-p`)

![screen shot 2013-07-18 at 7 42 27 pm](https://f.cloud.github.com/assets/120919/823205/53becef4-f005-11e2-8948-bea5122b419b.png)


Closes #466