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`
Makes sense, I wasn't sure which location made more sense. In this case it doesn't matter as much just because these won't be installed when the gem is installed, only when running bundle in the project.