set up simplecov

I had to change a bunch of `require`s to `require_relative` to resolve
pathing issues when running this. You can run the coverage with
`ruby test/test_all_basic.rb`, then open up `coverage/index.html`
Better to use a `.simplecov` file to configure SimpleCov, especially since we run our tests on travis via Rake. That way, SimpleCov will be configured and started consistently regardless of it being required in the Rakefile or elsewhere.