Add minimal Gemfile referencing gemspec

Also adds a '.gitignore' file.
Ensure *.dice templates are found
Add aruba gem as development dependency

Also generally tidied up the gemspec and added cucumber support
infrastructure.
Add basic feature file covering README scenarios

Updates .gitignore to cover tmp directory.
Add deprecation warning for non-`.dice` files
Merge pull request #47 from mdsol/feature/add_tests

Add scenario testing infrastructure