logicutils: add library authored by Peter Norvig

• logicutils.py is taken from logic.py that was originally distributed as part of the aima-python project, at http://code.google.com/p/aima-python/. At the time it was integrated into CDS Invenio (2010 May 20), aima-python had a notice on its website declaring the code to be under the "MIT license" and linking to http://www.opensource.org/licenses/mit-license.php by way of explanation.