pktout: add pktOut to SDN_Types

Represents a version-agnostic packet_out message from the OpenFlow spec.
pktout: fix port range check in OF0x01
pktout: implement from_packetOut function for OF0x01

Translates an SDN_Types.pktOut message to a OF0x01.packetOut messages.
pktout: make arg order for from_group consistent across versions
pktout: change packetOut representation for OF0x04

The port_id in a packet_out message should always be a physical port.
pktout: fix port range check for OF0x04
pktout: implement from_packetOut function for OF0x04

Translates an SDN_Types.pktOut message to a OF0x01.packetOut messages.
pktout: add payload_bytes function

Convenience function for getting the bytes out a payload
Merge pull request #119 from frenetic-lang/pktout

Add packet out to abstraction layer