tweet_rate fixed for Python 3.3

A couple of trivial fixes to get tweet_rate.py to run with Python 3.3.
First off, the urllib import statement and the urlencode funtion call
(urlencode now lives in urllib.parse). Secondly, the contents of
response.body (a 'bytes' object) needs to be decoded before it's passed
to json.loads(). Any other changes relate to Twitter's API.