Landcover recolour

While working on #565 I realized that the landuse colours are generally without reason. As some examples

* Lightness is all over the place
* Of the "base" landuses only ``landuse=retail`` has a border, but it's strong-red, only it's got a width of 0.3 so you don't realize it.
* ``landuse=retail`` is more closely related to shop POIs than ``landuse=commercial``, but ``landuse=commercial`` has a colour more similar to the shop colours

As a background, you can work in several different colour systems when picking colours. The best ones to work in are perceptual colour systems like CIELAB, CIELUV, and the angular variants. Unfortunately carto does not support these systems, so you need to convert to/from them. Fortunately, there are [websites](http://colormine.org/convert/rgb-to-lch) that will do that for you, and [websites with colour pickers](http://www.virtuosoft.eu/code/jquery-colorpickersliders/).

This pull request adjusts the colours for residential, retail, commercial, industrial/railway, farm/farmland, and farmyard. Exact details in the commits.

I've placed a demo up at http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405. Coverage is British Columbia, Washington, Northeast US, Tokyo, and Krakow.

Vancouver has all of the adjusted landuse within it, and one good example is in New Westminster, where there are Residential, Commercial, Retail and Industrial landuses near each other: http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#16.00/49.2/-122.92

Industrial
[![image](https://cloud.githubusercontent.com/assets/1190866/3159758/cf6fe0a4-eb15-11e3-9fc8-96a5b0d00090.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#16.00/49.2121/-122.8552)

Retail/Commercial (New to the right, Riverside Plaza is Retail, Riverside Business Center is Commercial)
[![image](https://cloud.githubusercontent.com/assets/1190866/3159789/41cd208a-eb16-11e3-9112-9aafae3d3e1c.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#16.00/48.4348/-122.3345)

Retail/Commercial
[![image](https://cloud.githubusercontent.com/assets/1190866/3159807/a5d12a0e-eb16-11e3-8164-acc175ddeabf.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#17.00/49.26290/-123.10324)

Residential
[![image](https://cloud.githubusercontent.com/assets/1190866/3159820/d3b628ca-eb16-11e3-8f56-e1b7e1d0bd42.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#18.00/49.18960/-122.93441)

Residential
[![image](https://cloud.githubusercontent.com/assets/1190866/3159877/c2cd1518-eb17-11e3-834a-b147ab382582.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#18.00/49.22054/237.07023)


Low-zoom Industrial
[![image](https://cloud.githubusercontent.com/assets/1190866/3159827/ed72bc10-eb16-11e3-9c8b-038749687509.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#12.00/49.1621/-122.9318)

Farms
[![image](https://cloud.githubusercontent.com/assets/1190866/3159839/183e8fdc-eb17-11e3-9bdb-b02d116e56ca.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#16.00/49.0784/-122.7118)

Many farm fields
[![image](https://cloud.githubusercontent.com/assets/1190866/3159894/f4145852-eb17-11e3-89da-fae354b13c77.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#15.00/49.0768/236.8546)

Low-zoom farms
[![image](https://cloud.githubusercontent.com/assets/1190866/3159848/36e3a9f4-eb17-11e3-853d-ce98370a0ad8.png)](http://bl.ocks.org/pnorman/raw/f1b113ca40658de27405/#11.00/49.0488/-122.6937)