Fix stem.doctest failures

I run doctest for stemmers and got the following failures:

    $ python -m doctest nltk/test/stem.doctest
    **********************************************************************
    File "nltk/test/stem.doctest", line 53, in stem.doctest
    Failed example:
        stemmer.stem("hurried")
    Expected:
        'hurri'
    Got:
        u'hurri'
    **********************************************************************
    File "nltk/test/stem.doctest", line 56, in stem.doctest
    Failed example:
        stemmer.stem("advisable")
    Expected:
        'advis'
    Got:
        u'advis'
    **********************************************************************
    File "nltk/test/stem.doctest", line 69, in stem.doctest
    Failed example:
        print(" ".join(SnowballStemmer.languages))
    Expected:
        danish dutch english finnish french german hungarian italian
        norwegian porter portuguese romanian russian spanish swedish
    Got:
        danish dutch english finnish french german hungarian italian norwegian porter portuguese romanian russian spanish swedish
    **********************************************************************
    1 items had failures:
       3 of  23 in stem.doctest
    ***Test Failed*** 3 failures.

The first and second failures were caused because doctest did not expect unicode output. The last failure was caused by missing NORMALIZE_WHITESPACE option.

This pull request resolves these issues and now stemmers can pass all tests.