New port: finance/py-python-bitcoinlib

Provides an easy interface to the bitcoin data structures and protocol. The
approach is low-level and "ground up", with a focus on providing tools to
manipulate the internals of how Bitcoin works.

WWW: https://github.com/petertodd/python-bitcoinlib/

Approved by: miwi (mentor)
Differential Revision: https://reviews.freebsd.org/D19542