Search should use unicode() instead of str()

See error below

```
File "/app/.heroku/python/lib/python2.7/site-packages/datatables/__init__.py", line 138, in _json
    query = self.search_func(query, str(search["value"]))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe7' in position 4: ordinal not in range(128)
```

Only ascii search terms work currently