Add OFPVID_PRESENT and OFPVID_NONE values for matching on VLAN tags

These values are used to indicate special conditions in matching
VLAN tagged frames.
(Motivated by FlowForwarding#134).
Merge pull request #55 from FlowForwarding/issue134

Add OFPVID_PRESENT and OFPVID_NONE values for matching on VLAN tags