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/124687/4251253/ca90dd5c-3a8b-11e4-9861-96d89e323e88.PNG)

This widget is essentially an alternative to #34, which provides a JSON API that includes similar information. The difference is that this doesn't require extra code (e.g. a Wordpress plugin) to be written by API clients, and it also allows us to quickly iterate on the widget without needing to update clients.