Netmon refactor fix

[jtpereyda]

Problem

When running network_monitor.py on Ubuntu, it would crash while trying to create the usage text.

Solution

create_usage was using global variables IFS and device when they weren't yet declared, and using device where it should have been pcapy_device. The first commit was the minimal fix.

The second commit sought to eliminate these global variables and clean up a couple warnings. Change summary:

• new get_ifs() method makes IFS variable available to main
• create_usage() now takes IFS as an input
• IFS renamed to ifs (PEP8)
• added main() method so that main code executes in a local scope. This addressed shadowing issues with the ifs variable.
• added required packages as suggested by PyCharm
• added docstrings

create_usage() is now side-effect free and we're using less global variables!

Tested

on Ubuntu. Verified docstring works and that network_monitor.py starts and accepts connections. I haven't figure out how to get Tornado on Windows yet so I can't test it there.