Python 3.3 fixes for tweet_rate examples

A few trivial fixes to get tweet_rate.py, tweet_rate_async.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().