blub
simple_switch_13.py: Added ability to use buffer_id in FlowMod

This avoids to send two packets (FlowMod & PacketOut).

OF v1.3.1 (the specification most switch vendors implemented) mentions in A.3.4.1 on page 65 that this is a valid way to avoid two packets.
Merge branch 'flowmod'

Merge for using buffer_id in FlowMod