provide OFPRR_* constants in FlowRemoved namespace
provide OFPPF_* constants in Port namespace
provide OFPFW_* constants in Match namespace
provide OFPR_* constants in PacketIn namespace
Add external attribute to output.
Merge pull request #240 from infrastation/develop

map more OpenFlow constants into Ruby space
Merge branch 'develop' into topo-trema-C
Merge branch 'topo-trema-C' into topo-trema-Ruby