Initial commit of an example firewall policy

Added firewall-policies.csv as an example with only one policy
Added pox_firewall.py to the repo

Added pox_firewall.py to the repo. This is the code for the
firewall written using pox.
Added more code to be nice

Added a little more code to make this easier for the students.
Otherwise, it was easy to get difficult to interpret error
messages
Added pyretic_firewall.py

Added pyretic_firewall.py, the firewall code template for pyretic.
Merge pull request #1 from ben-jones/assignment-7

Assignment 7