add Gemfile

This allows us to use bundler with literati.
make tests work with bundler

With this commit, the test suite runs under bundler:

  bundle install
  bundle exec rake test
remove contest gem dependency

This gem is deprecated upstream. Remove it, and replace it with ordinary
Minitest::Spec functions.
Merge pull request #12 from ktdreyer/bundler-and-minitest

add bundler and switch to minitest