Hi!

To allow logical expressions of keywords I've introduced
a reversed polish notation in the keyword indices. Attached
patch keyword-expressions.diff applies against rev. 4543

The idea:

Currently the keywords are represented like "keyword=4,5,42"
with an implicited OR operator of the listed keyword indices.

I defined three special indices (should be okay):

  -2: NOT
  -3: AND
  -4: OR

If the id sequence contains values lower than -1 it is
interpreted as an RPN expression with the special ids
as operators. e.g.

  4,5,-2,-3,42,-4 is read as (4 AND NOT 5) OR 42

If there are no ids below -1 the original behavior (ORed)
is used.

This notation is choosen to reduce the overall impact
to the code and it should be compatible with the old
behavior. Furthermore RPN allows complex expressions
without the need to introduce something like brackets.

The new feature is currently only available by modifying
the URLs. An editor inside Roundup would be nice.
I crafted a Javascript based demo expr-editor.html 
how this could work.
I've to look closer into the UI code of Roundup to
integrate this. If somebody could help me here ...
assistance is wellcome. :-)

Regards,
    Sascha