java_gen: add virtual getVersion method to all OFObjects
java_gen: create DEFAULT instance for objects that support it (for Match)

Objects which consist only of parts that have a default value
now get a default value.

Match, in particular, now defines a default instance that wildcards
everything and is available as factory.matchWildcardAll()
Merge into master from pull request #81:
a bunch of smaller improvements (https://github.com/floodlight/loxigen/pull/81)