Mcast forward

Hi,

I rebased the mcast-forward branch on snapshot 872366b5dfbcb2d8dd2a74f6e9d9889d735bdd2a of the cc-ports branch, it's available as 60bb775e188103238fc2eaad65e5313b55d0ff07. 

I managed to successfully compile the provided mcast example under examples/ip6 (with mcast define set to RPL). Feel free to pull mcast-forward back into your repository.

Regards,
Floris
edit: the Pull requests page seems to indicate that I want to merge the commits into g-oikonomou:cc-ports, I actually meant them to be merged into g-oikonomou:mcast-forwards.