revised classes cofinstructions and cofinst
renamed class: cofinst => cofinstruction
new test: cofinstruction_test
cofinstructions::unpack() => added missing OFPIT_EXPERIMENTER part
cofinstructions::unpack() => added missing OFPIT_EXPERIMENTER part
cofinstructions::unpack() => added missing OFPIT_EXPERIMENTER part
cofinstructions => added methods for managing generic instructions