Metric refactor

John, Patrick - This will change the return from /api/score.  JSON might be similar enough, but I haven't tried running your code yet, so the breakage is unknown.  I plan to merge and ship tonight, so tell me if you need the live site to stay stable today.

In short, the URLs are becoming real, and we'll start integrating live census data this week.

/api/score is a bit different.  Citations are dropped entirely, replaced with a 'detail_uri'.  The detail URI points to /api/detail/\<boundary_id\>/\<node_id\>/, that serves a GeoJSON representation of the boundary plus metric.  Boundary IDs are shorter.  Fake boundary slugs have changed, and now the fake boundaries are returned at /api/boundaries as well.

The biggest changes are backend.  Algorithms are almost entirely different.  Joe hasn't issued a pull request, so I expect a painful merge this week.  I'll start moving more logic into the database, so you might be doing a lot of `git pull; ./manage.py loaddata scores` in the future.