using LWPCookieJar insted FileCookieJar due to not implemented save()

I got following error:

```
>>> import simplemediawiki
>>> api = simplemediawiki.MediaWiki("http://example.com/api.php", cookie_file="cookie.txt")
Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
        File "/home/hamano/virtualenv/local/lib/python2.7/site-packages/simplemediawiki.py", line 113, in __init__
          self._cj.save()
        File "/usr/lib/python2.7/cookielib.py", line 1753, in save
          raise NotImplementedError()
      NotImplementedError
```

Perhaps we should use cookielib.LWPCookieJar insted cookielib.FileCookieJar.
Thank you.