Parking icon

Fixes #607

You can't really see the difference at low-resolution, but when producing a high resolution image the difference is visible

New:
![new_parking](https://cloud.githubusercontent.com/assets/1190866/3210323/fec9fd48-eeb3-11e3-827e-d1e180f0997a.png)

Old:
![old_parking](https://cloud.githubusercontent.com/assets/1190866/3210324/0012d0c6-eeb4-11e3-8757-28255f22efd4.png)

16x resolution version
![parking p 256](https://cloud.githubusercontent.com/assets/1190866/3210327/144e0f92-eeb4-11e3-88fb-a784871bed98.png)

@gravitystorm, should I add the 256px version anywhere?