a bunch of smaller improvements

Reviewer: @yotamhc @vaterlaus

- add getVersion as a virtual member to all OFObjects
- create a DEFAULT instance for objects that support it (i.e., all of whose members have a default)
- in particular, the DEFAULT match instance is 'wildcard all' and is available via factory.matchWildcardAll() (and the default value for classes that expect a match)
- fixed a bug with the default value of OFPort in OFMatchV1 (OF10)