Added serverspec to cookbook & app generators

chef generate will now automatically create a serverspec test for the default suite when it generates a .kitchen.yml.
- Updated app_spec.rb to check for serverspec suite
- Updated cookbook_spec.rb to check for serverspec suite
- Updated app.rb recipe to generate serverspec test for default suite
- Updated cookbook.rb recipe to generate serverspec test for default suite
- Added file for serverspec_spec_helper.rb
- Added template for serverspec_default_spec.rb