Rewrite buildings code

This is a relatively small change in terms of code, but substantial in what it impacts.

Fixes/superceeds

Railways:
#389
#327

Outlines:
#68
#490
#533

Supermarket
#520

Comments in this repository have shown that the light/dark distinction of buildings is not something easily understood, let alone something someone will grasp when viewing the map.

## Comparisons to other providers
I reviewed a number of other map providers, and most use something very subtle at low zooms and moderately subtle at higher zooms, and in all cases, less obtrusive than the current rendering.

Lyrk
![Lyrk](https://cloud.githubusercontent.com/assets/1190866/3076123/41698fec-e3b5-11e3-950e-ca6ea6490eb5.png)

MapQuest
![MapQuest](https://cloud.githubusercontent.com/assets/1190866/3076124/68ca5256-e3b5-11e3-93bf-428f6c588d25.png)

Here.com
![Here](https://cloud.githubusercontent.com/assets/1190866/3076126/81431c6e-e3b5-11e3-9d9c-f4798b4a9c40.png)

Google
![image](https://cloud.githubusercontent.com/assets/1190866/3076128/8ff0cd2e-e3b5-11e3-99b3-af353a02de50.png)

Bing
![Bing](https://cloud.githubusercontent.com/assets/1190866/3076133/b96b29d8-e3b5-11e3-8fac-6180c7912f0b.png)![Bing High](https://cloud.githubusercontent.com/assets/1190866/3076131/af544fb0-e3b5-11e3-9e69-52d794f99c6b.png)

Mapbox
![Mapbox](https://cloud.githubusercontent.com/assets/1190866/3076134/e5941704-e3b5-11e3-880e-b61cc919182d.png)![mapbox-low](https://cloud.githubusercontent.com/assets/1190866/3076135/ecf95522-e3b5-11e3-8415-3e8efe656693.png)

## Samples

![image](https://cloud.githubusercontent.com/assets/1190866/3076145/c79f6d60-e3b6-11e3-8715-1daefb1ee2aa.png)

![image](https://cloud.githubusercontent.com/assets/1190866/3076147/0bbe08e4-e3b7-11e3-85ec-ec9bd232029b.png)

![image](https://cloud.githubusercontent.com/assets/1190866/3076151/32712f34-e3b7-11e3-87ea-7141a7cebdc8.png)

## Outstanding issues

Low-zoom industrial buildings are too subtle
![image](https://cloud.githubusercontent.com/assets/1190866/3076159/984914e8-e3b7-11e3-93c0-ab6a6530c9b2.png)

I may decrease the zoom at which the outlines appear and slightly strengthen the fill, but the solution here might be to fade back the ``landuse=industrial`` fill to be more in line with residential/retail/commercial.

Roads under buildings are invisible.
![image](https://cloud.githubusercontent.com/assets/1190866/3076164/dfe0f4c4-e3b7-11e3-8a0a-1d6c4e3ffaa7.png)

Not a new issue. I'm tempted to punt on this. It also holds true for tunnels
![image](https://cloud.githubusercontent.com/assets/1190866/3076165/31ea75ba-e3b8-11e3-8126-64225b266b55.png)

## Churches (and other place_of_worship)
In keeping with more modern tagging of the ``amenity=place_of_worship`` on **either** the area or combined with ``building=*`` on the church building, the grey is moved to landcover. The gray has also been lightened slightly, depending on the original colour which depends on background.

amenity on area, building on buildings
![image](https://cloud.githubusercontent.com/assets/1190866/3076183/5e134840-e3ba-11e3-9d18-a4650cd0b502.png)

amenity and building on buildings
![image](https://cloud.githubusercontent.com/assets/1190866/3076187/c7fc1e3a-e3ba-11e3-9418-6da9455b7cf7.png)

Place of worship, no building
![image](https://cloud.githubusercontent.com/assets/1190866/3076188/33bdbef8-e3bb-11e3-9c0e-05231c6ba1f4.png)

## Railway stations
Railway stations were strange before. They've been fixed in line with https://github.com/gravitystorm/openstreetmap-carto/issues/327#issuecomment-34156070. Unfortunately, I can't find any great examples in the areas I've loaded.

## Demos

I've loaded up a portion of the northeast United States and made a comparison at http://bl.ocks.org/pnorman/raw/c61d6b11193081910866

Please keep in mind that the rendering server is less powerful, has a limited area, and is not consuming updates.

I have a small pre-rendered area from Seattle at http://a.tiles.mapbox.com/v3/pnorman.openstreetmap-carto/page.html which is faster.