Fixed SFP REST API. Forgot to add new flow's DPID to entry2dpid map, thus flow removal never triggered a flow-del message to the switch. Also, added a serializer for OFFlowMod and for OFFlowModMap. The latter is for the double-map of switch-dpid --> map of flow-name --> OFFlowMod object. The former has been separated, since it a JSON serializer for OFFlowMod might come in handy in the future. A present limitation is that the JSON serializer does not use the same syntax as is used to insert a flow-mod via the SFP REST API. This might be confusing for people if they get different output than what they put in. It's a lot less work to implement the serializer as-is though. The next step will be to match the SFP syntax on a per-field basis in the OFFlowModSerializer class.