grok_tweets.py: etc/twitter/students.json handles don't start with @
grok_tweets.py: first attempt at limiting duplicated posts
grok_tweets.py: more seriously limit duplicate posts
grok_tweets.py: make sure the bitlink is as we expect it
grok_tweets.py: remove more separators
Make grok_tweets.py a bit less interactive

Also, deploy some sleeps to avoid pissing of websites.