Improve and refactor routing setup

Move the routing code from mitm.__main__ to mitm.util.routing and make
the setup code much cleaner.

Also moves the rules to chains as they should be, throwing the rules
into nat PREROUTING and mangle PREROUTING is unfriendly to other things
on the system.
Merge pull request #1 from google/iptables_cleanup

Improve and refactor routing setup