add `icons` field to rocketbar search API endpoint (bug 1029635)

https://bugzilla.mozilla.org/show_bug.cgi?id=1029635

r? @robhudson

# v1
![](http://cl.ly/image/3z090m3K450Q/Screen%20Shot%202014-06-27%20at%207.01.03%20PM.png)

# v2
![](http://cl.ly/image/043D371d0g1N/Screen%20Shot%202014-06-27%20at%207.01.33%20PM.png)

usually when a method is a `classmethod` the first arg is named `cls` for clarity.