[Cooja] TrafficVisualizerSkin modifications

One of the major contributions of this PR is the visualization of sent but unreceived messages.
I've found this useful to detect interference and mote positioning problems when testing networks.

The other modifications focus on minor performance and aging issues and use alpha value-based fadeout.

![visualizer-skin](https://cloud.githubusercontent.com/assets/1416326/3840367/88bf570e-1e1c-11e4-9b76-6e3a8e43a58a.png)