Adds a Headless Radio Logger plugin for COOJA

This is a new COOJA plugin enabling the generation of a PCAP in headless mode.
I'm submitting as a pull request because several people have been merging our branch to get this feature.
The documentation on how to use the plugin is in `README.txt`.

If you prefer not to merge this in the main repository, I can contribute to Contiki-Projects instead or leave it on our repo and add it to the Contiki-OS wiki.