[Cooja] Configurable serial socket plugin

The original plugins lack some exception handling and configurability that this pr tries to add:

- Allows to configure listen port for server, and host + port for client plugin
- Connection info in status bar
- Exception handling
- Some visual enhancements
- package structure

![cooja-serialsocket](https://cloud.githubusercontent.com/assets/1416326/4084422/1aff0036-2f09-11e4-9bb6-3ad7fde4b5c7.png)