Added a neighbors() method of models.NetworkSwitch.  I was never able to figure out how to handle the self-referential many-to-many relationships among the NetworkSwitches, so this method is how I resolved that.  It returns a tuple with an instance of the the neighbor switch, the current switch's port that is connected to the neighbor switch, and the cost of that link.

Eventually, we'll probably want to figure out how to do this by setting up the correct relationships as it would probably result in better performance.