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)

Note that DRF also calls this method (`icons` is a `SerializerMethodField` above) and when it does, it doesn't call it as a class method but as an instance method, so it's probably a bad idea to make it a `classmethod`. It will only work by chance, because we don't use `self` in the implementation atm.