Selection of date ranges for hits

From any hits page, allow the viewing of a few pre-cooked date ranges:

* Yesterday
* Last seven days
* Last 30 days
* All time

Support for single days and ranges in the `?period=YYYYMMDD` or 
`?period=YYYYMMDD-YYYYMMDD` querystring is available,
but there's no UI (yet) for a range. You can view a single day's traffic by 
clicking a point on the graph.