Merge pull request #359 from g-oikonomou/trickle-timer-stop

Add a method to stop running trickle timers
Check for DAO fwd loop early, before processing DAO options
Define a new ICMPv6 message type for Trickle Multicast

Pending IANA allocation, we currently use private experimentation
New uip_is_addr macros for multicast addresses
Multicast common header files
Multicast routing table manipulation

We store multicast routes in a separate table since we don't need
as much information as we need for normal routes
We now know how to select our source address for outgoing routable multicast datagrams
RPL Multicast group management with MOP 3
Implementation of the SMRF multicast engine
Implementation of the Trickle multicast engine
Include multicast engines in the build
Implement multicast engine hooks in the uIPv6 core

  - init()
  - process incoming multicast datagram
  - Pass ICMPv6 trickle messages to the engine
Multicast README
IPv6 Multicast Example
Multicast compile regression tests