Versioned apps

- Adds a timestamp version to app JSON
- Updates to apps yield a new app version
- Adds an endpoint to list all versions of an app
- Adds an endpoint to show the detailed app parameters of an app version
- Updating an app with a previous version specified in the JSON body effects a rollback (a new version is created with the specified version's configuration)
- Added new REST API docs
    - [`GET /v2/apps/{appId}/versions`](https://github.com/mesosphere/marathon/blob/wip-versioned-apps/REST.md#get-v2appsappidversions)
    - [`GET /v2/apps/{appId}/versions/{version}`](https://github.com/mesosphere/marathon/blob/wip-versioned-apps/REST.md#get-v2appsappidversionsversion)
    - [`PUT /v2/apps/{appId}`](https://github.com/mesosphere/marathon/blob/wip-versioned-apps/REST.md#example-version-rollback)