Python 3.3 fixes for the tweet_rate examples

Fixes to get tweet_rate.py, tweet_rate_oauth.py, and tweet_rate_gen.py
to run with Python 3.3.  Notably, 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().