Levenshtein exception when mixing unicode and strings

[Glench]

Not sure if there's anything this library should do about this, but when you add as such you get an exception:

>>> f = FuzzySet()
>>> f.add(u'balls')
>>> f['balls']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/glen/.virtualenvs/temp/lib/python2.7/site-packages/fuzzyset/__init__.py", line 49, in __getitem__
    results = self.__get(value, i)
  File "/Users/glen/.virtualenvs/temp/lib/python2.7/site-packages/fuzzyset/__init__.py", line 74, in __get
    for _, matched in results[:50]]
  File "/Users/glen/.virtualenvs/temp/lib/python2.7/site-packages/fuzzyset/__init__.py", line 88, in _distance
    distance = Levenshtein.distance(str1, str2)
TypeError: distance expected two Strings or two Unicodes