Orchestration

The intent of this WIP PR is to solicit feedback on the PHP API of Orchestration early and often.

The first set of commits to this PR will be non-functional code samples for Orchestration. The goal of this exercise to design the Orchestration PHP API from the perspective of a developer using this API. Please review these commits as they come in and provide feedback. This will ensure that the feedback is incorporated into future commits.

Once the PHP API is finalized via code samples, the next set of commits to follow will be documentation of the API. Specifically, these will be a Getting Started Guide (similar to https://github.com/ycombinator/php-opencloud/blob/master/docs/userguide/ObjectStore/README.md) and a Complete User Guide (similar to https://github.com/ycombinator/php-opencloud/blob/master/docs/userguide/ObjectStore/USERGUIDE.md). There will be links from the latter to the previously-committed code samples.

Once the documentation commits have been reviewed and finalized, the final set of commits to this PR will be the tests and actual implementation of the API, that is the various classes under the `OpenCloud\Orchestration` namespace.

Once the tests and implementation have been reviewed and finalized, this PR will no longer be WIP. At this point, it may be merged.

To summarize:
* [x] Code samples
* [x] Documentation
* [x] Unit tests and implementation
* [x] Testing code samples against implementation

/cc @gecampbell @jamiehannaford 
The preview operation is side-effects-free on the service end, in that no resources are created or modified. Yet, the response to the preview operation returns information that isn't populated in the `Stack` object prior to that call (e.g. `resources`). Therefore I had this operation call `populate` with the response.

As for returning the Guzzle response, I agree its not useful but its what other operations such as `create`, `update`, and `delete` do so I made `preview` do the same to keep the API consistent from the user's perspective.

What do you think?