Made some line-colors dependent on their polygon-fill

I worked on resolving issue #57.

Objects with the following tags have now a border-color, that depends on the polygons fill-color:

* leisure
 * swimming_pool
 * playground
 * track
 * pitch
* tourism
 * camp_site
 * caravan_site
 * picnic_site
* landuse
 * field
 * retail
* natural
 * field
* power
 * station
 * generator
 * sub_station
 * substation
* amenity
 * parking
 * place_of_worship
* aeroway
 * aerodrome
 * terminal
* building=*
* railway
 * station

The dependency of both colors was expressed by adding:
```
line-color: saturate(darken(@color, x%), y%)
```
with 40 <= x <= 60
and 0 <= y <= 30

In my opinion this improves both the appearance and maintainability, because there is one variable per selector to define the colors of polygon-fill and line-color.

What do you think: Is this approach desirable for this mapstyle, or should the line-color rather be left hardcoded?