Box2DDebugRenderer - correct color for contacts, some small changes

sometimes, contacts have a wrong color. 
before:
![before](https://f.cloud.github.com/assets/3791221/383455/37f16f24-a618-11e2-9218-4e6619e2c40b.png)
after:
![after](https://f.cloud.github.com/assets/3791221/383457/3fda01a6-a618-11e2-9104-07a29eb9a29c.png)