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
Probably redundant, but sticking to the convention elsewhere in the
test, where recipes & recipes/default.rb were both listed.

See also #266 which does the same pattern.