java_gen: separate VlanVid and OFVlanVidMatch + push version to 0.3.1

OF1.3 specifies the concept of a 'PRESENT' bit (0x1000) that gets
added to the 12 bit VlanVid to indicate presence of a tagged vlan.

While the standard is unclear, our reading is that this is only
relevant for use in match fields/oxms (not in actions).

This commit accordingly splits up VlanVid into
 - a new VlanVid class (which just represents the concept of a 12-bit
    VLAN tag, as specified by 802.1Q), and
  - OFVlanVidMatch (which represents the concepts of an OF1.3 VlanTag
    + Presence Bit, as spec'ed in OF1.3 for matches).

As this is an API-incompatible change, the artifact version has
been pushed to 0.3.1
Merge into master from pull request #114:
java_gen: separate VlanVid and OFVlanVidMatch + push version to 0.3.1 (https://github.com/floodlight/loxigen/pull/114)