Add ability to convert command requests into command object controller parameter

Depends on #4.

 * `Identity:FooBar` → `Surfnet\StepupMiddleware\Identity\Command\FooBarCommand`
 * `Identity:Ns.FooBar` → `Surfnet\StepupMiddleware\Identity\Command\Ns\FooBarCommand`

Merges into feature/event-sourcing master PR.
We can't just automatically put it in front of the ParamConverter, typehint `CommandInterface` (marker interface) and handle it based on a check for the interface? (If this is not clear, we'll discuss it tomorrow)