fix non-ascii character encoding problem

 tornado.web.RequestHandler.get_argument returns a unicode string.
  http://www.tornadoweb.org/documentation/web.html?#tornado.web.RequestHandler.get_argument

And urllib.urlencode converts unicode chars into string chars before percent-encoding them.
If an argument contains non-ascii characters, encoding(default charset is system dependent) results in UnicodeEncodeError.

Example query:
  http://HOST:8000/?q=%E3%83%88%E3%83%AB%E3%83%8D%E3%83%BC%E3%83%89