humanize() should return unicode instead of str

It works OK with ascii-locales like the default:

    >>> print u'Now: {}'.format(arrow.now().humanize())
    Now: just now

But doesn't work with non-ascii locales like Russian:

    >>> print u'Now: {}'.format(arrow.now().humanize(locale='ru_RU'))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xd1 in position 0: ordinal not in range(128)