Alternative to pnorman's building style

In #565, @pnorman presented a style that results in lighter buildings, which I think is an improvement. However, the lighter buildings were not visible enough on residential and industrial landuse. I have taken the liberty of making some changes to pnorman's style so that buildings look better on these types of landuse. (If there is no landuse, the rendering is the same as in #565.) In this way, there is no need to change landuse colors.

## Comparison to pnorman's style
Left is pnorman's style from #565, right is the changed style
### Buildings on industrial
![ind_pnormanbuildings](https://cloud.githubusercontent.com/assets/5209216/3142504/3923d834-e9c2-11e3-92cf-f5b94ac9570e.png) ![ind_vholtenbuildings](https://cloud.githubusercontent.com/assets/5209216/3142505/3c1a0c02-e9c2-11e3-8e21-5e18dd32dc44.png)

### Buildings on residential
![res_pnormanbuildings](https://cloud.githubusercontent.com/assets/5209216/3142501/25666ea6-e9c2-11e3-8b78-b2873f47adfd.png) ![res_vholtenbuildings](https://cloud.githubusercontent.com/assets/5209216/3142502/28f8ff98-e9c2-11e3-9840-21777e3663df.png)

### Buildings without landuse (no difference)
![empty_pnormanbuildings](https://cloud.githubusercontent.com/assets/5209216/3142497/050ba9e6-e9c2-11e3-9331-ae51b5fb3c46.png)  ![empty_vholtenbuildings](https://cloud.githubusercontent.com/assets/5209216/3142498/0a056cf2-e9c2-11e3-8b21-102d0baaf92f.png)