Use modern conventions for structuring gems.

- Clean up the gemspec
- Don't specify gems in Gemfile
- Add version file
Configure RSpec with modern idioms.

- Don't need to require specific path for spec_helper as it should be
  in the load path already
- Configure rspec options with .rspec file
- Mirror spec directories with implementation directories
- Add some additional gems to measure test coverage and assist in
  debugging
Switch to expect syntax in specs.
Add yard for API documentation.
Add active support.