Add unit tests and recipe rake task

Discussed in https://github.com/degica/komoju/issues/1

This pull request adds the following:

* Splits up `Oops::OpsworksDeployment` into `Oops::Client` and `Oops::Deployment`
* Adds unit tests for `Oops::Client` and `Oops::Deployment`
* New rake task for executing recipes e.g. `rake oops:recipe[hats,hats-sandbox,dotenv]`