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`
This is a misleading pattern to use.  The gem version is determined at the time to gem is built.  So, unless you're building multiple gems per ruby, this won't work as expected.