add secrets.yml to source control

* following rails 4.1 secrets.yml approach
change config keys to more easily map to ENV vars


Current conversations about this topic, including wether to add secrets.ymlto source control by default:

* https://github.com/rails/rails/pull/13388
* https://github.com/rails/rails/pull/13463

I expect to support something more closely to the [figaro](https://github.com/laserlemon/figaro)
gem in the future. For now I wait until Rails 4.1 is released.