Remove stroke from map markers

This greater increases map panning performance on Firefox.
Add small map markers last

This prevents larger markers from hiding smaller ones. This is a
stopgap until we can clump markers.
Add an offline mode that doesn't render any tiles

Allow testing without a connection to the Internet.
Make categories a bit more general

Keep an array of all categories and have them store the id of their
chart element. This allows just iterating through all categories
instead of maintaining lists of them everywhere.
Display stats charts in a grid

This should work a lot better for shorter screens.
Make small markers slightly larger
Fix some errors that occur when everything is filtered

Avoid division by zero inside D3.
Update map when victims are filtered

The map now accurately reflects filtered victims, changing colors and
marker sizes appropriately.
Update popup contents when changing filter
Hard-code chart widths for now

We want to avoid doing a lot of repeated access of offsetWidth to
prevent reflow.
Split collisions into markers

Markers are created during JSON generation and the UI also tracks them.
These replace "collision groups."
Make the Marker responsible for adding itself
Add years 2008 through 2011 to the Oakland database
Deal with more data a bit better

Make the markers small and also sort newer collisions first in the
popup.
Treat collisions without victims as "other" injuries