#203 Add PATCH support for updating app instances

This PR adds support for PATCH to update app instances. The PATCH method behaves exactly as the PUT used to behave in earlier version except that it will retain the original container if no new container is defined by the update. The PUT method now stops the running app with the given ID and replaces it with the new `AppDefinition`. If no app with the given id exists, the app will be created.

Fixes #221 
Fixes #203