Support the NOT operator

This is really a quite simple patch.  I noticed a problem, though: it seems
that operator precedence is off in the case of "x LIKE y AND NOT a LIKE b".
TODO: Investigate.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added the "CORRELATE QUERY" type of statement

This statement type takes two queries, and outputs what keywords are most
highly representative of the first query relative to the other.  It can be used
to find candidate indicators for machine learning.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>