Add avatar in notifications

This adds avatars for desktop notifications about new comments.

It uses the existing "Hide user avatars" preference, though I could see it
needed as two separate settings... I didn't add it since I didn't want to add yet another setting.

That's pretty much it. I've been using it for a week and it's pretty
nice :smile: 

NB: not sure what happens on a pre-Mavericks OS. I can add an OS version check if necessary.

![trailer-notification](https://cloud.githubusercontent.com/assets/159328/3472751/69d6dfe6-02cf-11e4-8212-c0d1eecff42b.png)