Changes to the OpenFlow 1.2 match framework  

### Changes

* On match unpack, insert the proper match tlv object (e.g, if the oxm field is
an inport, insert and inport object).
* Add a compare method to oxm_tlvs and match_list, because there was not how to test
if the match in messages like flow_stats or flow_removed were correctly sent by the switch.
* Parsing of value on match unpack does not return a string value, due to problems when comparing a message created in the controller side and the message sent by the switch.
* Improvements to the print format of oxm_tlvs, easing the debug of messages containing it.

Also, there is a minor fix for the groups test.