add IGMP snooping application

this set is the upgrade of the following patch:
http://www.mail-archive.com/ryu-devel@lists.sourceforge.net/msg04205.html

Changes v1 -> v2:

- reduce unnecessary constant
- support OpenFlow1.2/1.3

this application provides the simple example of IGMP snooping.

the module "igmplib" mainly offers 2 functions:
- listening on in the IGMP conversation between multicast routers and hosts
- emulating of the querier function of multicast servers
the former operates a switch as a snooping switch and controls transmission of an unnecessary multicasting packet.
the latter realizes the IGMP conversation in the environment without multicast routers.

the module "simple_switch_igmp" is a variation of "simple_switch".
the switch receives the "EventPacketIn" event instead of the "EventOFPPacketIn" event from the module "igmplib" in order to except IGMP.