grok_tweets.py: don't process all student handles

There are cases where a student is talking about what another
student has posted. In such cases, the code was saving the post
like both students authored it. This is wrong.

Until I have a better idea to disambiguate cases like the ones
described above, let's prompt the operator.
grok_tweets.py: get files from RSS cache
rss_fetch.py: remove ad-hoc filters
rss_fetch.py: make sure there is no query string
grok_tweets.py: deal with bitlink API failures
grok_tweets rss_fetch: make sure we always shorten http:// links
rss_fetch: by default process all blogs
rss_fetch.py: normalize link as much as possible

Otherwise there is good probability that what we download is
not what we get when using Twitter.
rss_fetch: store files using the format of grok_tweets.py
grok_tweets: adapt to new rss_fetch saved-data format
grok_tweets.py: fix rss_cache_find
rss/blogs.json: virality.it -> www.virality.it
grok_tweets: allow the operator to specify the RSS cache dir
grok_tweets: add three second pause so one can read the logs