Create bin/render to emit JSON optimized for rendering www

- Create bin/render script which reads CrimeDB JSON from a data
  directory and emits JSON optimized for rendering the www grid view
- The grid has -1 values for cells outside of the region and values >= 0
  for real counts
Remove un-necessary second argument to bin/render
Use grid.json to render the crime grid

- Use the pre-computed data from grid.json to render the crime grid; has
  the side-effect of not rendering cells from outside the region
- Compute percentiles based on the entire region, not just the visible
  area; keeps them the same when we pan around
- Remove the region outline. Maybe add it back when we clip cells to
  the boundary so that things look better?
- Remove a bunch of un-necessary helpers in viz-util.js
bin/render: Round when computing cell intersections; misc cleanup
bin/render: Skip and warn on crimes in cells outside region
Add bin/render to bin/update
bin/render: Add --time-{from,to} args for filtering crimes

- Add --time-{from,to} args for selecting which crimes to render in the
  grid; units are epoch seconds UTC
- Iterate over all CrimeDB files in the selected date range to find
  crimes to render
bin/update: Render grids for 2014 rather than defaults