Added bigrams

Added extraction of word bigrams from tweets.

Retweets no longer increase values in the WordScore table (though
previously unseen words are still added). Hopefully this will make the
scores more representative of actual language use and less sensitive to
retweeted typos.

Improved maintenance performance by only reducing TwitterUser scores for
active users.
Added support for Twitter AND filters

No database change is needed, just add keyword filters with
space-separated terms.