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
diff --git a/java_gen/pre-written/pom.xml b/java_gen/pre-written/pom.xml
index e93b6f5..e516b6e 100644
--- a/java_gen/pre-written/pom.xml
+++ b/java_gen/pre-written/pom.xml
@@ -10,7 +10,7 @@
 
     <groupId>org.projectfloodlight</groupId>
     <artifactId>openflowj</artifactId>
-    <version>0.3.0-SNAPSHOT</version>
+    <version>0.3.1-SNAPSHOT</version>
     <packaging>jar</packaging>
 
     <name>OpenFlowJ-Loxi</name>