Police force area boundaries

Since I wrote scripts last year to create police force area boundaries from other datasets, the boundaries for forces in England and Wales (and a very vague polygon for Northern Ireland) have been made available as KML files at http://data.gov.uk/dataset/police-force-boundaries-england-and-wales.

The polygons here appear to be much more accurate than the neighbourhood ones were. Only Derbyshire is invalid, and is easily fixed by simplifying (which removes just one point to make it valid) using the new fix_invalid_polygons option for the existing mapit_import script.

Even if police forces aren't needed for UK MapIt, it would be useful to have this import script in master for others to use.

These commits also add a few useful options to mapit_import, a couple of minor fixes, and increase the maximum length of Code.code as discussed by email before.