Add a widget that lists Hive members for a city

This adds an endpoint at `/widgets/` (optionally prefixed by city on multi-city installs) that documents available embeddable widgets hosted by the directory. Currently, the only one is a member list widget:

![capture](https://cloud.githubusercontent.com/assets/124687/4249936/bb817bea-3a7d-11e4-8570-ef0ffc6f480a.PNG)