0.9 endpoint registry

Please check this out in a temporary branch. Its a very quick tweak of the rf12decoder to take advantage of the 'instancing' nature of the new registration system, allowing a better binding of decoder and device (assuming you want to write - as in "v" or "c", which I proposed down the line moving out to a different mechanism anyhow).

Most of the tweaks were generic message "instance" tweaks anyhow, and would benefit the decoder by itself, however i did not spend very long at all as I wanted it to keep very similar to sevenw's code so he could see benefits.