core: Complain loudly about Python 2.6

POX has always required Python 2.7, but I get a lot
of problem reports from people running 2.6.  The
first thing I do is point out that they're running
an unsupported Python -- now POX will do this for me.
webcore: By friendlier to Python 2.6

We use logger.getChild(), which is available starting in
Python 2.7, which is fair since that's POX's requirement.
We now fall back to a dumb way of doing it which works
in 2.6.