Add Gemfile

Most all projects include a Gemfile nowadays. This trivial Gemfile, just pulls its dependencies from the gemspec. If someone runs `bundle install` then they can always use a valid version of the required gems when hacking on this gem. 

I added Gemfile.lock to the gitignore, so it can always pull the most up-to-date gems allowed by the gemspec when a new dev clones it.

Enjoy.