release 0.6

This release changes only the development, not run-time, behavior
  * Gemspec puts lib dir on load path (thanks to Kyle VanderBeek)
  * Add Gemfile (thanks to Kyle VanderBeek)
  * Add devel dependencies: rake and minitest (thanks to Kyle VanderBeek)
  * Add rake tasks for building and cleaning extension
  * Release task runs tests
fix warning about dependency versions in gemspec