Fixes #414 - integrates rubocop gem

Adds rubocop gem and fixes majority of issues noted by cop.
Interesting. I did not know you could do that in Ruby: https://github.com/bbatsov/ruby-style-guide#underscores-in-numerics

But the idea is to make the numbers more readable, while having them make sense, so, you should treat the underscore as you would normally use a comma, so, it should be `10_485_760` :)