Added COK Trader

[greenkevon]

Created a new Trader to pull rates from COK

[marcelmorgan]

@greenkevon thanks for this. Looks good. Just 2 notes:

1. Add a verification file for the expected rates. If you looked at the rspec output, you would see this:

  Traders COK
    # Create expected rates YAML for COK at spec/rates/cok.yml
    # ./spec/traders_spec.rb:6

1. Commit messages are usually done in the present tense. So "add" instead of "added" (minor)

[greenkevon]

Thanks Marcel. I made the updates.

[marcelmorgan]

Looking great @greenkevon. Merging...