Add a page with statistics on public bodies

Stastistics like "how many successful requests were made to
The Department of Metasyntax?" are too slow to calculate
on-the-fly for this page, so this commit adds:

 * Extra columns on public_bodies to store summary statistics

 * A rake task which should be run periodically to update
   those statistics columns.  (This will also update the
   existing info_request_count column, which it seems wasn't
   being updated, except at the point where the migration
   introduced it.)

If Javascript is not available, the summary statistics are
shown as tables.  If Javascript is available, graphs are
drawn with Flot.
Print out progress in update_public_bodies_stats
Also display the percentage of overdue requests

Calculating the percentage of overdue requests is rather more
costly, since in order to avoid duplicating the overdue logic
we need to call calculate_status on each InfoRequest.
Make the minimum requests for statistics configurable
Add excanvas for support of graph-drawing on IE8 and before

This is as suggested in the Flot documentation.