Improvement: File extension configurable in supervisord.conf in order to facilitate using debian defaults

We've seen pull request #29, but preferred to retain the current functionality (it still defaults to `/etc/supervisor/*.ini`).
Instead we decided to make the extension of included service files configurable, so you are able to configure supervisor in debian way (to use `/etc/supervisor/conf.d/*.conf`). We demonstrate how to do it in the README.md.

Also, this pull request bootstraps testing with rspec-puppet.


Kudos to @fungusakafungus