Install RuboCop and fix offenses

This PR installs [Rubocop](https://github.com/bbatsov/rubocop) and I've gone through and fixed most offenses. If you search for `rubocop:disable` you'll be able to see all of the cases in which offenses were ignored.

Which linters to enable/disable/etc is certainly up for debate. Below are links to each .yml config file:

[Default Config](https://github.com/bbatsov/rubocop/blob/master/config/default.yml)
[Local Config](https://github.com/tmilewski/fauna-ruby/blob/a98958f65b3cedb293bd1640a37df73321aac5ef/.rubocop.yml)

Let me know what you think!

Thanks!