Add support for sending multicast IP packets.

[mlesniew]

This commit extends the library to enable the transmission of multicast IP packets.

Multicast packets differ from unicast packets only in their destination IP address. The only missing part in the library to support multicast was the calculation of destination MAC addresses. Unlike unicast, there's no need for ARP protocol to find the destination MAC address; it can be derived directly from the destination IP address. This commit adds the missing code for that.

[JAndrassy]

the uIP documentation has

uIP supports broadcast packets as well as sending multicast packets

you want to add support for multicast with a change in uIP, but if sending multicast packets is not possible maybe the problem is in the Arduino wrapper part of this library.

uIP https://github.com/adamdunkels/uip