Enhance retrieve() to allow the programm the specify the method
grok_tweets.py: add force (-f) switch to bypass tweets cache
grok_tweets.py: expand links before prompting the operator
grok_tweets.py: sleep before resolving links with HEAD
Teach grok_tweets to deal with multiple students in a tweet
grok_tweets.py: move checks on the link after HEAD expansion

Instrumental to modify the code to process more than one blog post
link in a single tweet at a time.
grok_tweets.py: really download only after we know it's not dup
grok_tweets: now a tweet can link to more than one post
grok_tweets: make prefix global
grok_tweets: move process_tweet() below process_student_tweet()
grok_tweets: Make sure the logging level is sane and log on stdout