packet lib: improve readability of json

ex) ethernet.ethernet

before applying this patch:

    {'ethernet': {'ethertype': 2048, 'dst': 'ZmY6ZmY6ZmY6ZmY6ZmY6ZmY=', 'src': 'MDA6MDA6MDA6MDA6MDA6MDA='}}

after applying tis patch:

    {'ethernet': {'ethertype': 2048, 'dst': u'ff:ff:ff:ff:ff:ff', 'src': u'00:00:00:00:00:00'}}