Python 3.3 for Twitter examples

A couple of trivial fixes to get tweet_rate_gen.py to run with Python
3.3.  First off, the urllib import statement has changed, together with
the urlencode function 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.