Ability to set codec in MulticastRTP Asterisk channels

Here you can find a port for Asterisk trunk (rev. 377802) of this patch: http://lists.digium.com/pipermail/asterisk-dev/2011-September/051262.html.
This patch permits to set the codec of an outgoing multicast RTP stream via the variable MULTICAST_RTP_CODEC:

exten => 999,1,Answer()  
exten => 999,n,Set(MULTICAST_RTP_CODEC=alaw)  
exten => 999,n,Dial(MulticastRTP/basic/239.255.255.245:5555,,A(/var/lib/asterisk/moh/mymoh))