Change level crossing symbol

This removes the white background of the level crossing symbol. I left a small white halo, so that it stands out more on the grey railway background. I also converted the symbol to a svg file.

Example renderings:
Before:
![tram_zoom15_before](https://cloud.githubusercontent.com/assets/5783139/4435662/956990c4-4751-11e4-820c-6904e85e4551.png)
After:
![tram_zoom15](https://cloud.githubusercontent.com/assets/5783139/4435661/9557d758-4751-11e4-8340-aef6c9720931.png)
![tram_zoom18](https://cloud.githubusercontent.com/assets/5783139/4435666/f072d926-4751-11e4-986f-1032ee865624.png)
![level_crossing_zoom14](https://cloud.githubusercontent.com/assets/5783139/4435667/f485f494-4751-11e4-9eab-4af5a3763f93.png)
![level_crossing_zoom16](https://cloud.githubusercontent.com/assets/5783139/4435668/f513876e-4751-11e4-80da-cb7af86aed39.png)