avatar stack

One design I liked in the [Whitney tearsheet mockup](https://f.cloud.github.com/assets/231252/2152587/376201d0-941b-11e3-84fe-8685cec1391b.png) is the 'stack', and I'm wondering if there's interest in setting up a similar thing for avatars - particularly groups, where we have this kind of thing in member profile:

![screen shot 2014-03-10 at 5 33 03 pm](https://f.cloud.github.com/assets/1885153/2379457/98e74228-a89b-11e3-9ae4-46194c795419.png)

We could totally clean up and consolidate that with a shared stack-y style.