Multicast Cooja regression tests

This patch adds two simple regression tests for the IPv6 multicast code - basically just simple linear node configurations that ensure that basic multicast forwarding works. The tests use the example code in `examples/ipv6/mcast6`.

![image](https://f.cloud.github.com/assets/589970/1274432/9e364fdc-2da1-11e3-87c8-d821740e260c.png)

(And a small code style commit.)