README updated
Changes to support MeterMod

To support the the meter modification message following chnages were made in addition to made by Raphael.

1) Corrected FLAG values on basis of OpenFlow specification.
2) Added commands supported by MeterMod.
3) Added new parameter 'id' to MeterMod as required by rfproxy for switch and datapath extraction.
4) Corrected return message for MeterMod.
5) Added Meter_Mod to RFProtocolFactory.
6) Added required imports to RFProtocol, RFServer and RFProxy.

Signed-off-by: Srijan Mishra <srijan23mishra@gmail.com>