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.
I created a `Command` interface. I believe we agreed on that. :)