Python 3.3 fixes for Twitter examples

A couple of trivial fixes to get tweet_rate.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.