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.
libopenflow: Fix ofp_phy_port _validate()

This is a regression caused by a recent improvement.
libopenflow: Use more symbolic constants

Many things were using hardcoded literals.
libopenflow: Minor cleanup
libopenflow: Better default for port_stats_request

The port was defaulting to 0, but should be OFPP_NONE.
libopenflow: Improve adding of port_stats

port_stats objects can be added as an easy way of summing the counters.
Previously, this lost the port_no value.  Now, it is retained when it
is the same in both the left and right operand.
Merge pull request #57 from noxrepo/betta

Pull from betta