Python 3.3 fixes for Twitter examples

A few trivial fixes to get tweet_rate.py to run with Python 3.3.  The
urllib import statement has changed, together with the urlencode
function call (urlencode now lives in urllib.parse). Also, 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.