Get library under Rubocop

[etscrivner]

Currently when running rubocop against bitcoin-ruby there are 9,662 offenses found with the default rules. It would be good to get this number to 0 and begin enforcing styling on all new PRs.

Since this is an extremely large number of offenses, it's unlikely that they could all be addressed at once. Rather they should be addressed in a series of PRs probably addressing the styling issues in individual files or logical file groupings first.