Replace parking icon with SVG

A lovingly handcrafted SVG by expert artisans, shaped by hand and optimized for map rendering use.

Okay, I wrote it by hand.

![image](https://cloud.githubusercontent.com/assets/1190866/3324964/dbb611f2-f787-11e3-9086-81300ceff8b8.png)

For compairson, current PNG: 
![image](https://cloud.githubusercontent.com/assets/1190866/3324968/fef97e10-f787-11e3-991c-44d6a924792b.png)

Gridded reference
![image](https://cloud.githubusercontent.com/assets/1190866/3324981/5bd50a5a-f788-11e3-94d2-f036846693b6.png)

Needless to say, it looks far better at high resolution. It also reduces the number of parking icons by one and allows the colour to be defined in the MSS.

@gravitystorm, I'm not sure about including ``marker-height`` in the MSS for reasons outlined in #642, but I've done so for now.