refs #1 Sentence.analysis_textでのUnicodeErrorを解決

Sentence.analysis_textでUnicodeErrorが発生する問題に当たりました::

    Traceback (most recent call last):                                                                                                                                                                                                 
      File "markovtest.py", line 13, in <module>                                                                                                                                                                                       
        m.analyze_sentence(text)                                                                                                                                                                                                       
      File "/home/hirokiky/.virtualenvs/markov/local/lib/python2.7/site-packages/markovchains/markovchains.py", line 37, in analyze_sentence                                                                                           
        words = self._get_words(sentence + u'。')                                                                                                                                                                                      
      File "/home/hirokiky/.virtualenvs/markov/local/lib/python2.7/site-packages/markovchains/markovchains.py", line 83, in _get_words                                                                                                 
        sentence.analysis_text(text)                                                                                                                                                                                                   
      File "/home/hirokiky/.virtualenvs/markov/local/lib/python2.7/site-packages/extractword.py", line 50, in analysis_text                                                                                                            
        words.append(Word(unicode(res.surface), unicode(res.feature))) 

これの解法をプルリクエストしています。

環境
-------

Python=2.7.1+
Mecab==0.97
mecab-python==0.97
UbuntuServer==12.04