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`
I like to pretend rubygems functions like perl and runs the tests when you install the gem. (which it used to be able to do, but was removed https://github.com/rubygems/rubygems/issues/730#issuecomment-30020000 )

It's mostly relevant these days for CI, when there a gems like `pry` that I may want to use in local dev but don't need installed on CI, which would happen with `add_development_dependency`.