MessageStringGenerator does not defined.

    print Datum({"hoge": "huga"})

cause a error, 

    File "/usr/local/lib/python2.7/dist-packages/jubatus-0.5.0-py2.7.egg/jubatus/common/datum.py", line 62, in __str__
        gen = jubatus.common.MessageStringGenerator() 
    NameError: global name 'jubatus' is not defined

This patch fixes this problem.