python: upgrade to python3

Force PCAP on by default, and remove dependency from cmake

Fixes #34

Could probably use more testing

Tidy up shared/static variables

cmake: find PCAP correctly on both Linux and OS X

cmake: use homebrew version of libpcap by default

Fixes #15

Add forgotten cmake module to git
- This *may* have been part of my testing method

Use cmake modules to alter the git commit hash when rebuilding
- as opposed to re-configuring

Add cmake module not provided by crappier distros

Add missing FindPCAP script

Make header location more sensible

Add extra search paths for btbb includes

Build libbtbb and pcaptools using cmake