Adds support for receiving multicast packets on Windows and Mac

Adds support for multicast for Windows and Mac/Posix.

With multicast, clients choose to subscribe to packets by subscribing to a multicast group. The benefit over sending by broadcast, i.e. setting the packets to be received by everyone on the network listening to a specific port, is that it enables routers to use network bandwidth more economically, mostly useful when streaming lots of data. http://en.wikipedia.org/wiki/Multicast_address

In this implementation, listening to an IP address in the multicast range (224.0.0.0 through 239.255.255.255) automatically enables multicasting on the socket.

I also added a multicast receive test. You can use the existing OscSendTests with the new OscReceiveMulticastTest by specifying address 239.0.0.1 as 'destination' address.