Fixed SFP REST API. Forgot to add new flow's DPID to entry2dpid map, thus flow...

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.